[gcov/Darwin] Ensure external symbols are exported when using an export list

Make sure that symbols needed to implement runtime support for gcov are
exported when using an export list on Darwin.

Without the clang driver exporting these symbols, the linker hides them,
resulting in tapi verification failures.

rdar://45944768

Differential Revision: https://reviews.llvm.org/D55151

llvm-svn: 348187