[ELF][MIPS] Implement R_MIPS_GPREL16/R_MIPS_GPREL32 relocations

The R_MIPS_GPREL16 / R_MIPS_GPREL32 relocations use the following
expressions for calculations:
```
local symbol:  S + A + GP0 - GP
global symbol: S + A - GP

GP  - Represents the final gp value, i.e. _gp symbol
GP0 - Represents the gp value used to create the relocatable object
```
The GP0 value is taken from the .reginfo data section defined by an object
file. To implement that I keep a reference to `MipsReginfoInputSection`
in the `ObjectFile` class. This reference is used by the
`ObjectFile::getMipsGp0` method to return the GP0 value.

Differential Revision: http://reviews.llvm.org/D15760

llvm-svn: 256416