[libcxx] [test] Don't use header defines for detecting linking against a DLL

In clang-cl/MSVC environments, linking against a DLL C++ standard
library requires having dllimport attributes in the headers; this
has been used for detecting whether the tests link against a DLL,
by looking at the libc++ specific define
_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS.

In mingw environments, thanks to slightly different code generation
and a couple linker tricks, it's possible to link against a DLL C++
standard library without dllimport attributes. Therefore, don't
rely on the libc++ specific header define for the detection.

Replace the detection with a runtime test.

Differential Revision: https://reviews.llvm.org/D125922