[mips64][mcjit] Add N64R6 relocations tests and fix N64R2 tests

This patch adds a test for MIPS64R6 relocations, it corrects check
expressions for R_MIPS_26 and R_MIPS_PC16 relocations in MIPS64R2 test, and
it adds run for big endian in MIPS64R2 test.

Patch by Vladimir Radosavljevic.

Differential Revision: http://reviews.llvm.org/D11217

llvm-svn: 246311