[ELF][MIPS] Ignore 'hint' relocations like R_MIPS_JALR in the `scanRelocs` method

MIPS ABI has relocations like R_MIPS_JALR which is just a hint for
linker to make some code optimization. Such relocations should not be
handled as a regular ones and lead to say dynamic relocation creation.

The patch introduces new virtual `Target::isHintReloc` method, overrides
it in the `MipsTargetInfo` class and calls it in the `Writer<ELFT>::scanRelocs`
method.

Differential Revision: http://reviews.llvm.org/D16193

llvm-svn: 257798