PR27216: Only define __ARM_FEATURE_FMA when the target has VFPv4

Summary:
According to the ACLE spec, "__ARM_FEATURE_FMA is defined to 1 if
the hardware floating-point architecture supports fused floating-point
multiply-accumulate".

This changes clang's behaviour from emitting this macro for v7-A and v7-R
cores to only emitting it when the target has VFPv4 (and therefore support
for the floating point multiply-accumulate instruction).

Fixes PR27216

Reviewers: t.p.northover, rengolin

Subscribers: aemerson, rengolin, cfe-commits

Differential Revision: http://reviews.llvm.org/D18963

llvm-svn: 267869