[clang] [llvm] [ARM] __ARM_ARCH macro definition fix (PR #81493)

[Andrew Pinski via cfe-commits]

pinskia wrote:

Note the corresponding GCC bug is at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99312 (and there was a patch posted but never reviewed; https://gcc.gnu.org/pipermail/gcc-patches/2021-March/566322.html).

https://github.com/llvm/llvm-project/pull/81493