gas: sframe: const ptrs for args and local vars where applicable

Use const pointers for function arguments and local vars where
applicable.  'cfi_insn' contains the DWARF CFI instruction data which,
for the purpose of SFrame generation, is read-only.  Similarly, some
getter APIs and output related APIs now document their argument as
read-only.

While at it, also remove the ATTRIBUTE_UNUSED from argument xlate_ctx in
sframe_xlate_do_register () because the argument is indeed conditionally
used.