arm64: Shave off two instructions in exceptions
This patch shaves off up to two three instructions in
save_registers_head in exception.S for arm64, which would make more
space for instructions that could be added in CheriBSD.
This is done by:
- Combining pointer arithmetic with pre-incrementing STP instructions
- Removing the instruction that sets the frame pointer (x29) as its content is unused
Differential Revision: https://reviews.freebsd.org/D34631