Don't force symbols to be globals in .thumb_set.

We currently force symbols to be globals in .thumb_set. The intent
seems to be that given

.thumb_set foo, bar

we emit an undefined symbol to bar if it is never defined. The side
effect is that we mark bar as global, even if it is defined, which gas
does not.

Producing an undefined reference to bar is a general difference from MC and gas.
For example, given

a = b

gas will produce an undefined reference to b, MC will not. I would be surprised
if any code depends on this, but it it does, we should fix the general
difference, not special case .thumb_set.

llvm-svn: 207757