[libcxxabi] Omit dllimport in public headers in MinGW mode

This matches the corresponding change to libcxx headers in
dfa88927.

This avoids needing to define _LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS
when building libcxxabi tests, for two reasons:
- It defaults to normal linkage (as opposed to dllimport), fixing
  linking of a static library version of libcxx/libcxxabi
- It avoids issues with using dllimport declarations on symbols when
  including cxxabi implementations into the testcase itself

Differential Revision: https://reviews.llvm.org/D148441