Use shmat syscall for Linux implementation

This patch add a direct call to shmat syscall if it is supported by
kernel features.

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

	* sysdeps/unix/sysv/linux/alpha/syscalls.list (shmat): Remove.
	* sysdeps/unix/sysv/linux/arm/syscalls.list (shmat): Likewise.
	* sysdeps/unix/sysv/linux/generic/syscalls.list (shmat): Likewise.
	* sysdeps/unix/sysv/linux/hppa/syscalls.list (shmat): Likewise.
	* sysdeps/unix/sysv/linux/ia64/syscalls.list (shmat): Likewise.
	* sysdeps/unix/sysv/linux/microblaze/syscalls.list (shmat): Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/syscalls.list (shmat):
	Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-64/syscalls.list (shmat):
	Likewise.
	* sysdeps/unix/sysv/linux/x86_64/syscalls.list (shmat): Likewise.
	* sysdeps/unix/sysv/linux/alpha/kernel-features.h (__NR_shmat):
	Define to __NR_osf_shmat.
	* sysdeps/unix/sysv/linux/shmat.c (shmat): Use shmat syscall if it is
	defined.