Add MAP_SYNC from Linux 4.15.

This patch adds the MAP_SYNC macro from Linux 4.15 to various
bits/mman.h headers.  Note that this is *not* added to all
architectures: in Linux 4.15, this macro is only in
asm-generic/mman.h, and only some architectures' asm/mman.h include
the asm-generic file - the architectures not using the asm-generic
file will need their own values of MAP_SYNC allocated to support this
functionality (some of them also already have conflicting mmap flags
so the value there will have to be different from the generic
0x80000).  Specifically, for glibc architectures, alpha hppa mips
powerpc sparc tile lack allocations of values for MAP_SYNC.

Tested for x86_64.

	* sysdeps/unix/sysv/linux/aarch64/bits/mman.h [__USE_MISC]
	(MAP_SYNC): New macro.
	* sysdeps/unix/sysv/linux/arm/bits/mman.h [__USE_MISC] (MAP_SYNC):
	Likewise.
	* sysdeps/unix/sysv/linux/ia64/bits/mman.h [__USE_MISC]
	(MAP_SYNC): Likewise.
	* sysdeps/unix/sysv/linux/m68k/bits/mman.h [__USE_MISC]
	(MAP_SYNC): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/bits/mman.h [__USE_MISC]
	(MAP_SYNC): Likewise.
	* sysdeps/unix/sysv/linux/nios2/bits/mman.h [__USE_MISC]
	(MAP_SYNC): Likewise.
	* sysdeps/unix/sysv/linux/riscv/bits/mman.h [__USE_MISC]
	(MAP_SYNC): Likewise.
	* sysdeps/unix/sysv/linux/s390/bits/mman.h [__USE_MISC]
	(MAP_SYNC): Likewise.
	* sysdeps/unix/sysv/linux/sh/bits/mman.h [__USE_MISC] (MAP_SYNC):
	Likewise.
	* sysdeps/unix/sysv/linux/x86/bits/mman.h [__USE_MISC] (MAP_SYNC):
	Likewise.