Sparc: Refactor R_SPARC_13/R_SPARC_GOT13 handling and fix a bug referencing absolute symbol
https://reviews.llvm.org/D47136 did not correctly handle `ld [%i0 + abs], %o0; abs = 7`
To fix it and make fixup handling less hacky,
* Change TableGen MEMri to use simm13Op instead of i32imm
* Emit a fixup of kind fixup_sparc_13 in SparcMCCodeEmitter::getSImm13OpValue
* Convert fixup_sparc_13 to either R_SPARC_13/R_SPARC_GOT13 in getRelocType
This postpones 13/GOT13 decision to relocation generation, ensuring that
we suppress the relocation when referencing an absolute symbol, matching
gas.