[MIPS] Set STO_MIPS_MICROMIPS flag and less-significant bit for microMIPS symbols

microMIPS symbols including microMIPS PLT records created for regular
symbols needs to be marked by STO_MIPS_MICROMIPS flag in a symbol table.
Additionally microMIPS entries in a dynamic symbol table should have
configured less-significant bit. That allows to escape teaching a
dynamic linker about microMIPS symbols.

llvm-svn: 318097