[yaml2obj] - Allow setting the symbol st_other field to any integer.

st_other field of a symbol usually contains its visibility.
Other bits are usually 0, though some targets, like
MIPS can set them using the named bit field values.

Problem is that there is no way to set an arbitrary value now,
though that might be useful for our test cases.

In this patch I introduced a way to set st_other to any numeric
value using the new StOther field.
I added a test and simplified the existent one to show the effect/benefit

Differential revision: https://reviews.llvm.org/D66583

llvm-svn: 369742