[libc] port fpe and process-related unit test utils to hermetic mode

[SchrodingerZhu]

We now have all the functionalities to implement the utility. Process exiting test should be run in hermetic mode.

• implement pthread_sigmask:
  we will use syscall directly or sigprocmask.
• implement sigsetjmp/siglongjmp
  in [libc] implement sigsetjmp/siglongjmp for x86-64 #136072
• port FPE catcher
• port process death utils (WIP [libc] start porting process unit tests to hermetic mode #135124)

[llvmbot]

@llvm/issue-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

We now have all the functionalities to implement the utility. Process exiting test should be run in hermetic mode.

[RossComputerGuy]

We have this error in the nix packaging when trying to build the tests. This is similar to #135158.

libc> In file included from /build/libc-src-21.0.0-unstable-2025-04-09/libc/test/UnitTest/LibcTest.h:29:
libc> In file included from /build/libc-src-21.0.0-unstable-2025-04-09/libc/src/__support/CPP/string.h:12:
libc> In file included from /build/libc-src-21.0.0-unstable-2025-04-09/libc/hdr/func/free.h:18:
libc> In file included from /build/libc-src-21.0.0-unstable-2025-04-09/libc/hdr/stdlib_overlay.h:28:
libc> In file included from /nix/store/4kgdnwbxq9h8qm5ckrq2hdl59gzhjh62-gcc-14-20241116/include/c++/14-20241116/stdlib.h:36:
libc> In file included from /nix/store/4kgdnwbxq9h8qm5ckrq2hdl59gzhjh62-gcc-14-20241116/include/c++/14-20241116/cstdlib:41:
libc> In file included from /nix/store/4kgdnwbxq9h8qm5ckrq2hdl59gzhjh62-gcc-14-20241116/include/c++/14-20241116/aarch64-unknown-linux-gnu/bits/c++config.h:680:
libc> /nix/store/4kgdnwbxq9h8qm5ckrq2hdl59gzhjh62-gcc-14-20241116/include/c++/14-20241116/aarch64-unknown-linux-gnu/bits/os_defines.h:39:10: fatal error: 'features.h' file not found
libc>    39 | #include <features.h>
libc>       |          ^~~~~~~~~~~~
libc> 1 error generated.

[michaelrj-google]

Given there are several related bugs it might be useful to create a parent issue, similar to #124027 to make tracking progress easier.

[RossComputerGuy]

@michaelrj-google Done as #135265