[libcxx] [libcxxabi] Set flags for visibility when statically linking...

[libcxx] [libcxxabi] Set flags for visibility when statically linking libcxxabi into libcxx for windows

Previously, these had to be set manually when building each of the
projects standalone, in order to get proper symbol visibility when
combining the two libraries.

Differential Revision: https://reviews.llvm.org/D90021