For mips64 switch statements in subroutines could generate

within the codegen EK_GPRel64BlockAddress. This was not 
supported for direct object output and resulted in an assertion.

This change adds support for EK_GPRel64BlockAddress for 
direct object.

One fallout from this is to turn on rela relocations 
for mips64 to match gas.

llvm-svn: 162334