[mc] Fix ELF st_other flag.

ELF_Other_Weakref and ELF_Other_ThumbFunc seems to be LLVM
internal ELF symbol flags.  These should not be emitted to
object file.

This commit defines ELF_STO_Shift for the target-defined
flags for st_other, and increase the value of
ELF_Other_Shift to 16.

llvm-svn: 196440