This is to ensure this macro is compatible with both C and C++.

Prior to this change, we wouldn't build headers that aren't referenced
by other parts of the libc which would result in a build error during
installation. To address this, we make the header target a dependency of
the libc archive. Additionally, we also redo the install targets, moving
the install targets closer to build targets and simplifying the
hierarchy and generally matching what we do for other runtimes.

This header first appeared in 4.4BSD and is provided by a number of C
libraries including Newlib. Several of our embedded projects use this
header and so to make LLVM libc a drop-in replacement, we need to
provide it as well.

For the initial commit, we only implement singly linked variants (SLIST
and STAILQ). The doubly linked variants (LIST, TAILQ and CIRCLEQ) can be
implemented in the future as needed.

These were fixed properly by f1f1875c18b8.

- Revert "[libc] temporarily set -Wno-shorten-64-to-32 (#77396)"
- Revert "[libc] make off_t 32b for 32b arm (#77350)"

Fixes the following diagnostic:

    llvm-project/libc/src/sys/mman/linux/mmap.cpp:44:59: error: implicit
conversion loses integer precision: 'off_t' (aka 'long long') to 'long'
    [-Werror,-Wshorten-64-to-32]
     size, prot, flags, fd, offset);
                            ^~~~~~

It looks like off_t is a curious types on different platforms. FWICT,
it's 32b
on arm (at least for arm-linux-gnueabi) but 64b elsewhere (including 32b
riscv32-linux-gnu).

Summary:
A previous patch added a dependency on the stack protectors, this was
not built on the GPU targets so every test was disabled. It turns out
that disabled tests still get targets so we need to specifically check
if the it is in the target's set of entrypoints before we can use it.

Another patch, because the build-bot was down, snuck in that prevented
the new math tests from being run. The problem is that the `signal.h`
header requires target specific definitions but was being used
unconditionally. I have made changes that disable building this header
if the file is not defined in the config. This required disbaling the
signal_to_string utility, so that will simply be missing from targets
that don't define it.

When building with compiler-rt enabled, warnings such as the following
are
observed:


llvm-project/llvm/build/projects/compiler-rt/../libc/include/llvm-libc-macros/linux/sys-stat-macros.h:46:9:
    warning: 'S_IXOTH' macro redefined [-Wmacro-redefined]
    #define S_IXOTH 00001
            ^

llvm-project/llvm/build/projects/compiler-rt/../libc/include/llvm-libc-macros/linux/fcntl-macros.h:61:9:
    note: previous definition is here
    #define S_IXOTH 01
            ^
It looks like we have these multiply defined. Deduplicate these flags;
users
should expect to find them in sys/stat.h. S_FIFO was wrong anyways
(should
have been S_IFIFO).