Fix an out-of-bounds error in RegisterContextDarwin_arm64

Summary:
gcc diagnoses this as "array subscript 63 is above array bounds of
'RegisterContextDarwin_arm64::VReg [32]'".

The correct fix seems to be subtracting the fpu register base index, but
I have no way of verifying that this actually works.

Reviewers: jasonmolenda

Subscribers: javed.absar, kristof.beyls, lldb-commits

Differential Revision: https://reviews.llvm.org/D59495

llvm-svn: 357055