[libc][POSIX] Implement clock_settime

[amemov]

When I was looking at the project codebase and at the documentation, I noticed that clock_settime is not implemented:

llvm-project/libc/docs/headers/time.rst

Line 19 in 4e404d0

- `linux-x86_64 <https://github.com/llvm/llvm-project/tree/main/libc/config/linux/x86_64/entrypoints.txt>`_

I would like to do write. Please assign this issue to me. Thanks!

[llvmbot]

@llvm/issue-subscribers-libc

Author: Anton Shepelev (amemov)

When I was looking at the project codebase and at the documentation, I noticed that `clock_settime` is not implemented: https://github.com/llvm/llvm-project/blob/4e404d0e5135ae7d0d4e7cc4c9b04d456eb50a01/libc/docs/headers/time.rst#L19

I would like to do write. Please assign this issue to me. Thanks!