[ELF][MIPS] Fix .MIPS.options ri_gp_value on MIPS64

The VA of _gp was being truncated to 32 bits when calling getVa(), but
for 64bit MIPS we need to write a 64 bit value to .MIPS.options.

Patch by Alexander Richardson.

Differential revision: https://reviews.llvm.org/D27672

llvm-svn: 289432