[ELF] - Detemplate MipsGotSection<ELFT>

This continues detemplation process.

Detemplating MipsGotSection<ELFT> is helpfull because can
help to detemplate getRelocTargetVA. (one more change is required)
It opens road to detemplation of GotSection<ELFT> and probably
something else after that.

Differential revision: https://reviews.llvm.org/D31090

llvm-svn: 298272