ppc: Fix emulated single to double denormalized conversions

helper_todouble() was not properly converting any denormalized 32 bit
float to 64 bit double.

Fix-suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paul A. Clarke <pc@us.ibm.com>

v2:
- Splitting patch "ppc: Three floating point fixes"; this is just one part.
- Original suggested "fix" was likely flawed.  v2 is rewritten by
  Richard Henderson (Thanks, Richard!); I reformatted the comments in a
  couple of places, compiled, and tested.
Message-Id: <1566250936-14538-1-git-send-email-pc@us.ibm.com>

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>