[AArch64] Don't #define __ARM_FP when there's no FPU.

On some R-profile CPUs, leaving out the FPU is an option. Clang will
accept `-march=armv8-r+nofp`, but it's currently not possible to find
out via the preprocessor whether it's in that mode (e.g. to change or
disable inline asm statements in your code).

The __ARM_FP macro, which has a bit set for each size of floating
point number supported by the hardware, is the natural thing to test.
But Clang was defining it unconditionally on AArch64. Now it checks
for FP support before defining it at all.

Reviewed By: tmatheson, DavidSpickett

Differential Revision: https://reviews.llvm.org/D145781