This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

`man 3 signal`'s declaration has a face _only a mother could love_.

sighandler_t and __sighandler_t are not defined in the C standard, or POSIX.

They are helpful typedefs provided by glibc and the Linux kernel UAPI headers
respectively since working with function pointers' syntax can be painful. But
we should not rely on them; in C++ we have `auto*` and `using` statements.

Remove the proxy header, and only include a typedef for sighandler_t when
targeting Linux, for compatibility with glibc.

Fixes: #125598

- migrate more `-O3` to `${libc_opt_high_flag}`
- workaround a issue with `LLP64` in test. The overflow testing is
guarded by a constexpr but the literal overflow itself will still
trigger warnings.

Notice that for math smoke test, for some reasons, the
`${libc_opt_high_flag}` will be passed into `lld-link` which confuses
the linker so there are still some warnings leftover there. I can
investigate more when I have time.

added proxy header for the stack_t type and modified the corresponding
CMakeLists.txt files

Added proxy headers for __sighandler_t type, modified the corresponding
CMakeLists.txt files and test files

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.