[ELF][MIPS] Set GP0 value to zero in case of relocatable object generation

LLD does not update relocations addends when generate a relocatable
object. That is why we should not write a non-zero GP0 value into
the .reginfo and .MIPS.options sections. And we should not accept input
object files with non-zero GP0 value because we cannot handle them
properly.

llvm-svn: 282716