Dllexport stuff cleanup:

Dllexport stuff cleanup:
1. Emit external function type information for all COFF targets since it's
a feature of object format
2. Emit linker directives only for cygming (since this is ld-specific stuff)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@84214 91177308-0d34-0410-b5e6-96231b3b80d8