Make getBaseSymbol non recursive.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207759 91177308-0d34-0410-b5e6-96231b3b80d8
[ARM64] Prefer generation of bzero on Darwin only

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207760 91177308-0d34-0410-b5e6-96231b3b80d8
Move getBaseSymbol somewhere the COFF writer can use.

I will use it there in a second.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207761 91177308-0d34-0410-b5e6-96231b3b80d8
Compute the correct section for zed = foo + 1 in COFF.

This fixes pr19147.

There are a few more related issues to fix, but the testcase in the bug now
passes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207763 91177308-0d34-0410-b5e6-96231b3b80d8