__strong_alias is an ELF feature that is not available on Darwin/MachO.
We have openlibm_strong_reference to paper over these differences,
but we weren't using it consistently. In particular, we were still
using it to define long double -> double aliases on platforms where
these are the same type (e.g. aarch64). This didn't used to matter,
because the only such platform we supported was always Linux and
thus ELF. This patch fixes these instances thus fixing the build
on Apple Silicon.
This is a bit more consistent with the naming of the other header files
(openlibm_complex.h and openlibm_fenv.h). Re-add an openlibm.h header
that includes all of the public headers as a shorthand.
Fix up all of the source files to include <openlibm_math.h> instead of
<openlibm.h>. While there, fix ordering of the includes.
Keno Fischer