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.