tests/uid.test: adopt for alpha

Make the test work on alpha that has getxuid syscall instead of getuid.

* tests/uid.awk (BEGIN): Update getuid regexp to match both getuid
and getxuid syscalls.
* tests/uid.c (main): Allow __NR_getxuid as an alternative
to __NR_getuid.
* tests/uid.test: If getuid syscall is not available, probe for
getxuid syscall.
3 files changed