[MIPS] Add MIPS64 R6 relocations

This patch adds 6 MIPS64 R6 relocations:

- R_MIPS_PC21_S2
- R_MIPS_PC26_S2
- R_MIPS_PC18_S3
- R_MIPS_PC19_S2
- R_MIPS_PCHI16
- R_MIPS_PCLO16

It also adds missing R_MIPS_PC16 and R_MIPS_32.

Change-Id: I89601a93e4c1b9b40e81edc3c84474377fb98eb6
1 file changed