[libc][cmake] Enable assertions and Werror for baremetal builds (#214093)

Enable LLVM_ENABLE_ASSERTIONS and LLVM_ENABLE_WERROR in
baremetal_common.cmake
to ensure pre-merge checks catch assertion failures and compiler
warnings for
baremetal targets.

Assisted-by: Gemini
GitOrigin-RevId: 723eaa4f83396c81212784fb0006e3ca4be19441