make the addend fixup code a bit more generic

Patch by Jack Carter.

llvm-svn: 145998

- lower unaligned loads/stores.
- encode the size operand of instructions INS and EXT.
- emit relocation information needed for JAL (jump-and-link).  

llvm-svn: 145113

"With this patch we can now generate runnable Mips code through LLVM
direct object emission. We have run numerous simple programs, both C
and C++ and with -O0 and -O3 from the output. The code is not production
ready, but quite useful for experimentation." Patch and message by
Jack Carter

llvm-svn: 144414

Patch by Reed Kotler at Mips Technologies.

llvm-svn: 140878