[ELF][MIPS] Always resolve MIPS GP-relative relocations to 'local' definitions

In case of MIPS, GP-relative relocations always resolve to a definition
in a regular input file, ignoring the one-definition rule. Such
relocations are used to setup GP relative offsets in a function's
prologue. So we, for example, should not attempt to create a dynamic
relocation even if the target symbol is preemptible.

Fixes bug 27880.

Differential Revision: http://reviews.llvm.org/D20664

llvm-svn: 271100