Centralize the handling of the thumb bit.

This patch centralizes the handling of the thumb bit around
MCStreamer::isThumbFunc and makes isThumbFunc handle aliases.

This fixes a corner case, but the main advantage is having just one
way to check if a MCSymbol is thumb or not. This should still be
refactored to be ARM only, but at least now it is just one predicate
that has to be refactored instead of 3 (isThumbFunc,
ELF_Other_ThumbFunc, and SF_ThumbFunc).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207522 91177308-0d34-0410-b5e6-96231b3b80d8