[cmake] [ARM] Exclude any VFP builtins if VFP is not supported

Summary:
rL325492 disables FPU features when using soft floating point
(-mfloat-abi=soft), which is used internally when building for arm. This causes
errors with builtins that utililize VFP instructions.

With this change we check if VFP is enabled (by checking if the preprocessor
macro __VFP_FP__ is defined), and exclude such builtins if it is not enabled.

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

Reviewed By: peter.smith

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

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

llvm-svn: 338284