alpha: take kernel_execve() out of entry.S

Signed-off-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Michael Cree <mcree@orcon.net.nz>
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>