* libc/include/sys/features.h: Redefine compilation environment
definitions for Cygwin to cover 64 bit Cygwin.
* libc/ctype/ctype_.c (_ctype_): Fix definition for 64 bit Cygwin.
* libc/include/machine/setjmp.h: Change definition of _JBLEN to allow
different values for 32 bit and 64 bit Cygwin.
* libc/include/reent.h (stat64): Define as stat under Cygwin, instead
of as __stat64. Undef stat64 if not building Newlib.
* libc/include/sys/stat.h (stat64): Define as stat under Cygwin.