[builtins][ARM] Check __ARM_FP instead of __VFP_FP__.

This patch fixes builtins' CMakeLists.txt and their VFP tests to check
the standard macro defined in the ACLE for VFP support. It also enables
the tests to be built and run for single-precision-only targets while
builtins were built with double-precision support.

Differential revision: https://reviews.llvm.org/D92497