[Patch][Aarch64] Implement Aarch64 SIMD ABI and aarch64_vector_pcs attribute

[Steve Ellcey]

I have a question about my own patch.  In doing testing I realized
that if I use the aarch64_vector_pcs attribute on a platform without
SIMD (i.e. -march=armv8.1-a+nosimd) I get an ICE.  That is obviously
not what we want but I was wondering what the right behaviour is.

We certainly don't want to generate code for a SIMD function on a
target that does not support SIMD, but is it OK to declare something
with the simd or aarch64_vector_pcs attribute if it is never used or called?
Or should even the declaration of a function with the simd/aarch64_vector_pcs
attribute result in a compilation error?  I don't think we want to
complain about simd declarations because those could show up in the system
headers like mathcalls.h even when not used.  Or should those be ifdef'ed
in some way based on the __ARM_NEON macro so they don't show up when not
compiling for SIMD?

Any ideas on where should this check be done, I thought the
TARGET_OPTION_VALID_ATTRIBUTE_P hook might be the right place, but that
seems to be specific to the 'target' attribute only, not attributes
in general.

Steve Ellcey
sellcey@cavium.com