[ELF][MIPS] Support linking of PIE for MIPS

MIPS ABI requires creation of the MIPS_RLD_MAP dynamic tag for non-PIE
executables only and MIPS_RLD_MAP_REL tag for both PIE and non-PIE
executables. The patch skips definition of the MIPS_RLD_MAP for PIE
files and defines MIPS_RLD_MAP_REL.

The MIPS_RLD_MAP_REL tag stores the offset to the .rld_map section
relative to the address of the tag itself.

Differential Revision: https://reviews.llvm.org/D43347

llvm-svn: 329996