util: Implement PIC assembly for the aarch64.

util: Implement PIC assembly for the aarch64.

When accessing m5_mem and building PIC code, we need to get the address
of m5_mem out of the global offset table, and then load the value from
there. If we try to load from m5_mem directly, the assembled code has a
relocation type the linker can't handle when building a shared object.

Change-Id: Ieb19c3d17c37ef810559ee24b68886b18ddcc869
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27212
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>