unwindstack: RegsArm64: fix harmless off by 1 + iterate GPRs contiguously

The memcpy should be for 31 GPRs, [x0, x30]. Currently it (accidentally)
also copies over the SP register (which ends up being harmless, as the
layouts match, and the value is reassigned again anyway).

Separately, I'm including an optional change for the iteration order,
since LR is the x30 GPR, it makes slightly more sense to print it
immediately after x29. However, this is a change in behaviour, so I can
undo the change if you think it's not worth it.

Tested: atest libunwindstack_unit_test
Change-Id: Ib6b81f8ee3a9a526bfabe4b09b327f083c855fb8