Don't read off end of GPR register array to access fp/sp/lr/pc

The arm64 register context on Darwin has the 29 general purpose
registers, then pc/sp/lr/fp with different field names depending
on compile-time flags.  Instead of accessing beyond the end of
the uint64_t[29] array, and upsetting the sanitizers, access those
registers correctly with the correct name.

Fixes a test failure on the ASAN CI bot, currently being
skipped, in TestEarlyProcessLaunch.py.

Differential Revision: https://reviews.llvm.org/D140067
rdar://103359354