hoist handling of external globals and special globals up to common code.

hoist handling of external globals and special globals up to common code.
This makes a similar code dead in all the other targets, I'll clean it up
in a bit.

This also moves handling of lcomm up before acquisition of a section,
since lcomm never needs a section.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93851 91177308-0d34-0410-b5e6-96231b3b80d8