[libc] Implement timer_getoverrun in time (#234839)

### Summary
Implement the standard POSIX function `timer_getoverrun` in `<time.h>`.

- Adds internal Linux syscall wrapper in
`libc/src/__support/OSUtil/linux/syscall_wrappers/timer_getoverrun.h`.
- Implements `timer_getoverrun` entrypoint in
`libc/src/time/linux/timer_getoverrun.cpp`.
- Declares `timer_getoverrun` in `libc/include/time.yaml`.
- Registers entrypoints for Linux (`x86_64`, `aarch64`, `riscv`).
- Adds unit test in `libc/test/src/time/timer_test.cpp`.

Implementing `timer_getoverrun` finishes the POSIX timer API in LLVM
libc.

Fixes #234067.

GitOrigin-RevId: 48c35e1861f64ff4e42197aa21380b635193e21e