Cygwin: sys/cpuset.h: add cpuset-specific external functions

Cygwin: sys/cpuset.h: add cpuset-specific external functions

The latest incarnation of sys/cpuset.h broke building coreutils.
The reason is the inclusion of stdlib.h and string.h and hence
premature requests for datatypes not yet defined in the include
chain.

Avoid this by defining __cpuset_alloc and __cpuset_free as external
functions, now defined in sched.cc.  Linux is doing this too, just
using different names for the functions. Redefine  __cpuset_zero_s
to use __builtin_memset only on compilers supporting it, otherwise
using a simple loop. Drop the stdlib.h and string.h includes.

Fixes: 3f2790e04439 ("Cygwin: Make gcc-specific code in <sys/cpuset.h> compiler-agnostic")
Reported-by: Denis Excoffier <cygwin@Denis-Excoffier.org>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>