The ARM C Language Extenstion document defines __ARCH_ARM as the integer
macro indicating the current ARM instruction set.  Let's use it.

PR/55778: evbarm64 GENERIC64 kernel fails to build with clang

XXX Handle the fact that for an ARM architecture ARMvX.Y then,
XXX __ARM_ARCH= X * 100 + Y. E.g. for ARMv8.1 __ARM_ARCH = 801.

(skrll)