shm_open2: Implement SHM_GROW_ON_WRITE

Lack of SHM_GROW_ON_WRITE is actively breaking Python's memfd_create tests,
so go ahead and implement it. A future change will make memfd_create always
set SHM_GROW_ON_WRITE, to match Linux behavior and unbreak Python's tests
on -CURRENT.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D25502

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

@@ -190,6 +190,7 @@
  * shmflags for shm_open2()
  */
 #define	SHM_ALLOW_SEALING		0x00000001
+#define	SHM_GROW_ON_WRITE		0x00000002
 
 /*
  * Flags for memfd_create().