alpha: improve testsuite

misc tiny patches:
* add a 'check' target to the Makefile
* split code in crt.s to create the _exit syscall; also use the value of
main() as exit status

Signed-off-by: Tristan Gingold <gingold@adacore.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5244 c046a42c-6fe2-441c-8c8c-71466251a162
