CodeGen: support dll{ex,im}port on WoA

Add ARM support for dllexport and dllimport attributes.  This is a relatively
conservative change.  The alternative is to entirely drop the architecture
requirement.  The dllimport and dllexport attributes are not restricted to any
architecture, simply to platforms that support this attribute (currently
Windows).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208486 91177308-0d34-0410-b5e6-96231b3b80d8