[AMDGPU] Use absolute relocations when compiling for AMDPAL and Mesa3D (#67791)

The primary ISA-independent justification for using PC-relative
addressing is that it makes code position-independent and therefore
allows sharing of .text pages between processes.

When not sharing .text pages, we can use absolute relocations instead,
which will possibly prevent a bubble introduced by s_getpc_b64.

Co-authored-by: Thomas Symalla <thomas.symalla@amd.com>