[libc++] Change workaround for init_priority((100)) outside of system headers

We had previously been using a different workaround for pretending that
we were inside a system header, however it had some undesirable effects
on dependency parsing for build systems, as explained in [1].

This patch changes the workaround to use `#pragma GCC system_header`,
which shouldn't suffer from the same issue. Unfortunately, it is a lot
more verbose. The issue is that `#pragma GCC system_header` is ignored
when we are inside a source file, so we have to create a header just for
the sake of using it. IMO this seems like an artificial restriction
without much benefit, but investigating that is a different story.
For now, this should at least solve build system problems at the
cost of some readability.

[1]: https://reviews.llvm.org/D95972#3178968

Differential Revision: https://reviews.llvm.org/D115334