__always_inline is a (Free)BSD specific definition from cdefs.h.
Compilers like Clang don't care about these keywords anyway, as they
simply do whatever they like.
It looks like we already removed this keyword from ld80/e_rem_pio2l.h as
well. Apply exactly the same change to the ld128 version as well.
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.
Put external headers before internal ones. While there, replace a lot of
occurences of "openlibm.h" with <openlibm.h>. It should be thought of as
an external header, as it is installed along with the library.
