alpha: fix stat64 issue

The stat64/fstat64 syscalls are broken for alpha linux-user.

This is because Alpha, even though it is native 64-bits, has a stat64
syscall that is different than regular stat.  This means that the
"TARGET_LONG_BITS==64" check in syscall.c isn't enough.  Below is
a patch that fixes things for me, although it might not be the cleanest
fix.

This issue keeps sixtrack and fma3d spec2k benchmarks from running.

Signed-off-by: Vince Weaver <vince@csl.cornell.edu>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
