[PECOFF] Allow symbols not starting with '_' in x86

Not all symbols are decorated with an underscore in x86. You can
write undecorated symbols in assembly, for example. Thus this
assertion is too strong.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207125 91177308-0d34-0410-b5e6-96231b3b80d8