[ELF2][mips] Support both big and little endian MIPS 32-bit targets

- Make the `MipsTargetInfo` template class with `ELFType` argument. Use
  the argument to select an appropriate relocation type and read/write
  routines.
- Add template function `add32` to add-and-write relocation value in
  both big and little endian cases. Keep the `add32le` to reduce code
  changes.

Differential Revision: http://reviews.llvm.org/D13723

llvm-svn: 250297