[cmake] [ARM] Check if VFP is supported before including any VFP builtins

Summary:
rL325492 disables FPU features when using soft floating point
(-mfloat-abi=soft), which is used internally when building for armv7. This
causes errors with builtins that utililize VFP instructions. With this change
we first check if VFP is enabled (by checking if the preprocessor macro
__VFP_FP__ is defined) before including such builtins.

Reviewers: rengolin, samsonov, compnerd, smeenai, javed.absar, peter.smith

Reviewed By: peter.smith

Subscribers: peter.smith, mgorny, kristof.beyls, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D47217

llvm-svn: 333216