[ARM] Copy-paste error in ARMv87a architecture definition.

In the tablegen architecture definition, the Name field for the
ARMv87a record read "ARMv86a". All the other records contain their own
names.

Corrected it to "ARMv87a", and added the necessary value in
ARMArchEnum for that to refer to.

Reviewed By: pratlucas

Differential Revision: https://reviews.llvm.org/D96493