alpha: Wire up all missing implemented syscalls

And still left the missing unimplemented syscalls as warnings. The
related warnings for missing implemented syscalls:

    CALL    scripts/checksyscalls.sh
  <stdin>:1241:2: warning: #warning syscall getrandom not implemented [-Wcpp]
  <stdin>:1244:2: warning: #warning syscall memfd_create not implemented [-Wcpp]
  <stdin>:1250:2: warning: #warning syscall execveat not implemented [-Wcpp]

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>