alpha: unbreak OSF/1 (a.out) binaries

OSF/1 brk(2) was broken by following one-liner in sys_brk()
(commit 4cc6028d

):

-	if (brk < mm->end_code)
+	if (brk < mm->start_brk)
		goto out;

The problem is that osf_set_program_attributes()
does update mm->end_code, but not mm->start_brk,
which still contains inappropriate value left from
binary loader, so brk() always fails.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>