[libc++] Only build with -fvisibility=hidden on Clang

The visibility annotations in libc++ are not quite right for GCC, which
results in symbols not being exported when -fvisibility=hidden is used.
To fix the GCC build bots, this commit reverts to the previous state of
not building with hidden visibility on GCC.

In the future, we can build with hidden visibility all the time and
export symbols explicitly using a list. See https://llvm.org/D66970
for one take at this.

llvm-svn: 370926