See PR #132823 for a previous change to sys/epoll tests, and prior
changes before that.

ErrnoCheckingTest ensures that errno is properly reset at the beginning
of the test case, and is validated at the end of it. This change removes
many instances of manual errno manipulation.

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`.

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.

The ErrnoSetterMatcher target was renamed in a previous patch, but not
all uses were caught. This patch fixes those that remain.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Along the way, couple of additional things have been done:

1. Move `ErrnoSetterMatcher.h` to `test/UnitTest` as all other matchers live
   there now.
2. `ErrnoSetterMatcher` ignores matching `errno` on GPUs.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151129

`getrandom` is implemented as a syscall.
We don't want to test linux implementation of the syscall. We just want to verify that it reacts as expected to sensible values.

Runtime before
```
[ RUN      ] LlvmLibcGetRandomTest.InvalidFlag
[       OK ] LlvmLibcGetRandomTest.InvalidFlag (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.InvalidBuffer
[       OK ] LlvmLibcGetRandomTest.InvalidBuffer (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.ReturnsSize
[       OK ] LlvmLibcGetRandomTest.ReturnsSize (took 83 ms)
[ RUN      ] LlvmLibcGetRandomTest.PiEstimation
[       OK ] LlvmLibcGetRandomTest.PiEstimation (took 9882 ms)
```

Runtime after
```
[ RUN      ] LlvmLibcGetRandomTest.InvalidFlag
[       OK ] LlvmLibcGetRandomTest.InvalidFlag (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.InvalidBuffer
[       OK ] LlvmLibcGetRandomTest.InvalidBuffer (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.ReturnsSize
[       OK ] LlvmLibcGetRandomTest.ReturnsSize (took 0 ms)
[ RUN      ] LlvmLibcGetRandomTest.CheckValue
[       OK ] LlvmLibcGetRandomTest.CheckValue (took 0 ms)
```

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D151336

Previously, only those unit tests which belonged to a suite were run as
part of libc-unit-tests. It meant that unit tests not part of any suite
were not being tested. This change makes all unit tests run as part of
libc-unit-tests. The convenience function to add a libc unit test suite
has been removed and add_custom_target is used in its place. One of the
bit-rotting test has along been fixed. Math exhaustive and differential
tests are skipped under full build.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148784

`getrandom` should return the number of bytes successfully set on
success, not `0`.

Reviewed By: sivachandra, michaelrj

Differential Revision: https://reviews.llvm.org/D147981