Implement .reloc (constant offset only) with support for R_MIPS_NONE and R_MIPS_32.

Summary:
Support for R_MIPS_NONE allows us to parse MIPS16's usage of .reloc.
R_MIPS_32 was included to be able to better test the directive.

Targets can add their relocations by overriding MCAsmBackend::getFixupKind().

Subscribers: grosbach, rafael, majnemer, dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D13659

llvm-svn: 252888