[ARM] Limit the diagnose when an ISR calls a regular function

Summary:
When the function is compiled with soft-float or on CPU with no FPU, we
don't need to diagnose for a call from an ISR to a regular function.

Reviewers: jroelofs, eli.friedman

Reviewed By: jroelofs

Subscribers: aemerson, rengolin, javed.absar, cfe-commits

Differential Revision: https://reviews.llvm.org/D32918

llvm-svn: 302274