alpha: Do not redefine __NR_shmat or __NR_osf_shmat

Fixes build using v5.1-rc1 headers.

The kernel has cleaned up how these are defined.  Previous behavior
was to define __NR_osf_shmat as 209 and not define __NR_shmat.
Current behavior is to define __NR_shmat as 209 and then define
__NR_osf_shmat as __NR_shmat.

	* sysdeps/unix/sysv/linux/alpha/kernel-features.h (__NR_shmat):
	Do not redefine.
	* sysdeps/unix/sysv/linux/alpha/sysdep.h (__NR_osf_shmat):
	Do not redefine.