ARM: allow .thumb_func to be separated from symbol definition

When assembling, a .thumb_func directive is supposed to be applicable to the
next symbol definition, even if there are intervening directives. We were
racing ahead to try and find it, and this commit should fix the issue.

Patch by Gabor Ballabas

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193403 91177308-0d34-0410-b5e6-96231b3b80d8