radeonsi/compute: Use relocs for scratch pointer rather than user sgprs v2
radeonsi/compute: Use relocs for scratch pointer rather than user sgprs v2

Instead of passing a pointer to the scratch buffer via user sgprs, we
now patch the shader with the buffer address using reloc information
from the LLVM generated ELF.

v2:
  - Make sure not to break older LLVM.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler