[mips][ias] Work around incorrect microMIPS relocation evaluation exposed by r268900

microMIPS has a special case that is not correctly implemented in LLVM. If we
have a symbol 'foo' which is equivalent to '.text+0x10'. The value of an
R_MICROMIPS_LO16 relocation using 'foo' is 'foo+0x11' and not 'foo+0x10'. The
in-place addend should therefore be 0x11.

Work around this by partially reverting the effect of r268900 by keeping the
symbol when the STO_MIPS_MICROMIPS flag is set. This fixes
SingleSource/Regression/C/PR640 for microMIPS.

llvm-svn: 269196