[RFA] Fix recent compilation errors in alpha-nat.c on alpha-osf

[Joel Brobecker]

> I'll commit this; it mimmicks what I did in the other Alpha code.
> 
> 	* alpha-nat.c (get_longjmp_target): Use ALPHA_* constants
> 	where needed.
> 	(fetch_osf_core_registers): Likewise.
> 	(supply_gregset): Likewise.

Thanks for the help. I relaunched the testsuite, and here are the
results on our Tru64 5.1A machine. They should be identical to
yesterday's results: 

# of expected passes            6990
# of unexpected failures        791
# of unexpected successes       3
# of expected failures          149
# of unresolved testcases       64
# of unsupported tests          2

-- 
Joel