Fix build of MiBench on FreeBSD RISC-V

Unlike other architectures the RISC-V headers on FreeBSD don't expose the
macros used here (since RISC-V does not provide floating-point traps).
This patch checks for the presence of FP_X_INV before using it.

Reviewed By: dim
Differential Revision: https://reviews.llvm.org/D89557
1 file changed