[libc] Normalize LIBC_TARGET_MACHINE

Current implementation defines LIBC_TARGET_MACHINE with the use of CMAKE_SYSTEM_PROCESSOR.
Unfortunately CMAKE_SYSTEM_PROCESSOR is OS dependent and can produce different results.
An evidence of this is the various matchers used to detect whether the architecture is x86.

This patch normalizes LIBC_TARGET_MACHINE and renames it LIBC_TARGET_ARCHITECTURE.
I've added many architectures but we may want to limit ourselves to x86 and ARM.

Differential Revision: https://reviews.llvm.org/D101524

GitOrigin-RevId: 7c2ece523d7ff74f3eeabce1b9685f3eaae8cff4
17 files changed