[llvm][AArch64] Actually check expected FPU for CPUs

We were passing this as an argument but never using
it. ARM has always checked this.

Note that the FPU list is shared between ARM and AArch64
so there is no AArch64::getFPUName, just ARM::getFPUName.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D93387