alpha: fix conversion from denormal float to double

The trap handler does properly update the fraction,
but not the exponent...

Thanks to Paolo Bonzini for the bug report and the testcase.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Paolo Bonzini <bonzini@gnu.org>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>