Rework handling of basename variants

Rework handling of basename variants

As a commonly-included header, the #define basename in <string.h> can
affect code which uses "basename" for its own purposes (e.g. struct
members or C++ namespaced functions).  When such cases occur and some
code includes <string.h> and some not, then errors result.  OTOH,
<libgen.h> is rarely used, and that's where the renaming occurs in
glibc, so code using <libgen.h> should already be safe.

* libc/include/libgen.h (basename): Define as __xpg_basename
for source compatibility with glibc.
Declare with __ASMNAME("basename") for ABI compatibility.
* libc/include/string.h (basename): Define as basename for
source compatibility with glibc.
Declare with __ASMNAME("__gnu_basename") for ABI compatibility.