Add MAP_SHARED_VALIDATE from Linux 4.15.

This patch adds the MAP_SHARED_VALIDATE macro from Linux 4.15 to
bits/mman-linux.h and the hppa bits/mman.h.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/bits/mman-linux.h [__USE_MISC]
	(MAP_SHARED_VALIDATE): New macro.
	* sysdeps/unix/sysv/linux/hppa/bits/mman.h [__USE_MISC]
	(MAP_SHARED_VALIDATE): Likewise.