MC: Let non-temporary COFF aliases be in symtab

MC was aping a binutils bug where aliases would default their linkage to
private instead of internal.

I've sent a patch to the binutils maintainers and they've recently
applied it to the GNU assembler sources.

This fixes PR20152.

Differential Revision: http://reviews.llvm.org/D4395

llvm-svn: 212899