Split arch_align_stack() out from asm-generic/system.h

Split arch_align_stack() out from asm-generic/system.h into its own header of
asm-generic/exec.h as part of the asm/system.h disintegration.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>