aarch64: Clarify the initial adjustments to sp for kernel exception entry

The first store-pair instruction in save_registers_head decrements the
stack pointer to create room for most of the trapframe (and the
redzone area for nested exceptions). However, the expression was a
bit non-intuitive as it was using an expression without a leading
negative sign to adjust the stack pointer. Rewrite the expression so
that it is more clearly a negative adjustment to the stack pointer.

Suggested by: andrew
Reviewed by: andrew
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D52514