Linking of shared libraries for MIPS little-endian 32-bit target.

The following are the most significant peculiarities of MIPS target:
- MIPS ABI requires some special tags in the dynamic table.
- GOT consists of two parts local and global. The local part contains
  entries refer locally visible symbols. The global part contains entries
  refer global symbols.
- Entries in the .dynsym section which have corresponded entries in the
  GOT should be:
  * Emitted at the end of .dynsym section
  * Sorted accordingly to theirs GOT counterparts
- There are "paired" relocations. One or more R_MIPS_HI16 and R_MIPS_GOT16
  relocations should be followed by R_MIPS_LO16 relocation. To calculate
  result of R_MIPS_HI16 and R_MIPS_GOT16 relocations we need to combine
  addends from these relocations and paired R_MIPS_LO16 relocation.

The patch reviewed by Michael Spencer, Shankar Easwaran, Rui Ueyama.
http://llvm-reviews.chandlerc.com/D2156