[ELF][MIPS] Add STO_MIPS_PLT flag to the symbols require pointer equality

On MIPS we need to mark symbol which has a PLT entry and requires
pointer equality by STO_MIPS_PLT flag. That is necessary to help
dynamic linker distinguish such symbols and MIPS lazy-binding stubs.

https://sourceware.org/ml/binutils/2008-07/txt00000.txt

Differential Revision: http://reviews.llvm.org/D17593

llvm-svn: 261879