[ELF][MIPS] Calculate relocations agains _gp_disp / __gnu_local_gp using dedicated 'expressions'

The patch introduces two new relocations expressions R_MIPS_GOT_GP and
R_MIPS_GOT_GP_PC. The first one represents a current value of `_gp`
pointer and used to calculate relocations against the `__gnu_local_gp`
symbol. The second one represents the offset between the beginning of
the function and the `_gp` pointer's value.

There are two motivations for introducing new expressions:
- It's better to keep all non-trivial relocation calculations in the
  single place - `getRelocTargetVA` function.
- Relocations against both `_gp_disp` and `__gnu_local_gp` symbols
  depend on the `_gp` value. It's a magical value points to the "middle"
  of GOT. Now all relocations use a common `_gp` value. But in fact,
  under some conditions each input file might require its own `_gp`
  value. I'm going to implement it in the future patches. So it's
  better to make `MipsGotSection` responsible for calculation of
  the `_gp` value.

llvm-svn: 298306