Fixes https://github.com/llvm/llvm-project/issues/100964

These changes slighly modify the output of the unittests so that they better
match GTest, so that utilities that parse the expected output from GTest (such
as Android's unit test harness) can read the output from our unit tests.

This allows our unit tests to be run on Android devices.

Add very primitive command line parsing to:
- support --gtest_color=no to disable printing terminal colors.
- recognize --gtest_print_time and print the test time in milliseconds.
  - most of our unit tests run on the order of microseconds, so its useful to
    preserve the existing behavior.  But upsteram GTest ONLY prints time tests
    in milliseconds, and Android's atest expects to be able to parse exactly
    that. Atest always passes --gtest_print_time. The word `took` is removed as
    that also differs from upstream GTest, tripping up parsers.
- ignore other --gtest_* flags

Do so so that atest can parse the output correctly.

Print the test number count before
each run, so that atest can parse this value correctly.

Link: https://android-review.googlesource.com/c/platform/external/llvm-libc/+/3107252
Link: https://google.github.io/googletest/advanced.html#colored-terminal-output
Link: https://google.github.io/googletest/advanced.html#suppressing-the-elapsed-time

Context: https://github.com/llvm/llvm-project/pull/87017

- Add proxy header `libc/hdr/math_macros.h` that will:
  - include `<math.h>` in overlay mode,
- include `"include/llvm-libc-macros/math-macros.h"` in full build mode.
- Its corresponding CMake target `libc.hdr.math_macros` will only depend
on `libc.include.math` and `libc.include.llvm-libc-macros.math_macros`
in full build mode.
- Replace all `#include "include/llvm-libc-macros/math-macros.h"` with
`#include "hdr/math_macros.h"`.
- Add dependency to `libc.hdr.math_macros` CMake target when using
`add_fp_unittest`.
- Update the remaining dependency.
- Update bazel overlay: add `libc:hdr_math_macros` target, and replacing
all dependency on `libc:llvm_libc_macros_math_macros` with
`libc:hdr_math_macros`.

Reland of #84991

A downstream overlay mode user ran into issues with the isnan macro not
working in our sources with a specific libc configuration. This patch
replaces the last direct includes of math.h with our internal
math_macros.h, along with the necessary build system changes.

Summary:
There are a lot of random training spaces on various lines. This patch
just got rid of all of them with `sed 's/\ \+$//g'.

Summary:
This is a massive patch because it reworks the entire build and
everything that depends on it. This is not split up because various bots
would fail otherwise. I will attempt to describe the necessary changes
here.

This patch completely reworks how the GPU build is built and targeted.
Previously, we used a standard runtimes build and handled both NVPTX and
AMDGPU in a single build via multi-targeting. This added a lot of
divergence in the build system and prevented us from doing various
things like building for the CPU / GPU at the same time, or exporting
the startup libraries or running tests without a full rebuild.

The new appraoch is to handle the GPU builds as strict cross-compiling
runtimes. The first step required
https://github.com/llvm/llvm-project/pull/81557 to allow the `LIBC`
target to build for the GPU without touching the other targets. This
means that the GPU uses all the same handling as the other builds in
`libc`.

The new expected way to build the GPU libc is with
`LLVM_LIBC_RUNTIME_TARGETS=amdgcn-amd-amdhsa;nvptx64-nvidia-cuda`.

The second step was reworking how we generated the embedded GPU library
by moving it into the library install step. Where we previously had one
`libcgpu.a` we now have `libcgpu-amdgpu.a` and `libcgpu-nvptx.a`. This
patch includes the necessary clang / OpenMP changes to make that not
break the bots when this lands.

We unfortunately still require that the NVPTX target has an `internal`
target for tests. This is because the NVPTX target needs to do LTO for
the provided version (The offloading toolchain can handle it) but cannot
use it for the native toolchain which is used for making tests.

This approach is vastly superior in every way, allowing us to treat the
GPU as a standard cross-compiling target. We can now install the GPU
utilities to do things like use the offload tests and other fun things.

Some certain utilities need to be built with 
`--target=${LLVM_HOST_TRIPLE}` as well. I think this is a fine
workaround as we
will always assume that the GPU `libc` is a cross-build with a
functioning host.

Depends on https://github.com/llvm/llvm-project/pull/81557

- [reland] #78192 
- [reland] #78447 
- Turn `as` static function into a `to_storage_type` member function.

Reverts llvm/llvm-project#78447
This broke the gcc buildbot.

- [reland] #78192
- Make the implementation work when `__uint128_t` is not available on
the plaftorm.