For mips64 switch statements in subroutines could generate

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162334 91177308-0d34-0410-b5e6-96231b3b80d8