The current tradition of openlibm is to hide all of its internal symbols
into the reserved system namespace. CloudABI has a check in place to
ensure that its C library (which contains openlibm) to not place any
unwanted symbols into the public namespace. openlibm seems to leak
_scan_nan() in there, so we'd better add an additional underscore.
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.
