[libc] Implement BSD extensions in <err.h> (#199055)
Implemented the BSD-extension functions defined in <err.h>:
- err, errx, verr, verrx
- warn, warnx, vwarn, vwarnx
Added an internal reporting helper in src/__support/Err. The err family
functions call internal::exit directly to satisfy [[noreturn]]
requirements. Enabled these entrypoints for aarch64, riscv, and x86_64.
Assisted-by: Automated tooling, human reviewed.
