sys/_bitset.h: Fix fall-out from commit 5e04571cf3c

The changes to the bitset macros allowed sched.h to be included into
userland programs without name space pollution due to BIT_* and
BITSET_* macros.

The definition of a global variable "bitset" had been overlooked.
This name space pollution caused a compile failure in print/miktex.

This commit renames the bitset variable to __bitset with the same
mapping back to the bitset if _KERNEL or _WANT_FREEBSD_BITSET is
defined.

This fix has been suggested by kib. It has been tested to let the
build of the print/miktex port succeed and to not break buildworld.

This commit shall be MFCed together with commit 5e04571cf3c.

Reported by:	arrowd
MFC after:	1 month

newlib/libc/sys/rtems/include/sys/_bitset.h

@@ -61,11 +61,13 @@ struct _t {								\
  * Define a default type that can be used while manually specifying size
  * to every call.
  */
-__BITSET_DEFINE(bitset, 1);
+__BITSET_DEFINE(__bitset, 1);
 
 #if defined(_KERNEL) || defined(_WANT_FREEBSD_BITSET)
 #define	BITSET_DEFINE(_t, _s)	__BITSET_DEFINE(_t, _s)
 #define	BITSET_DEFINE_VAR(_t)	__BITSET_DEFINE_VAR(_t)
+
+#define	bitset			__bitset
 #endif
 
 #endif /* !_SYS__BITSET_H_ */