Use shmdt syscall for linux implementation

this patch add a direct call to shmdt syscall if it is supported by
kernel features.

hecked on x86_64, i686, powerpc64le, aarch64, and armhf.

	* sysdeps/unix/sysv/linux/alpha/syscalls.list (shmdt): Remove.
	* sysdeps/unix/sysv/linux/arm/syscalls.list (shmdt): Likewise.
	* sysdeps/unix/sysv/linux/generic/syscalls.list (shmdt): Likewise.
	* sysdeps/unix/sysv/linux/hppa/syscalls.list (shmdt): Likewise.
	* sysdeps/unix/sysv/linux/ia64/syscalls.list (shmdt): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/syscalls.list (shmdt): Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/syscalls.list (shmdt):
	Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-64/syscalls.list (shmdt):
	Likewise.
	* sysdeps/unix/sysv/linux/x86_64/syscalls.list (shmdt): Likewise.
	* sysdeps/unix/sysv/linux/shmdt.c (shmdt): Use shmdt syscall if it is
	defined.