Improve -mpreferred-stack-boundary configure message.

Improve -mpreferred-stack-boundary configure message.

Explicitly say we are checking for -mpreferred-stack-boundary=2.
Only 2 is ever used when it is supported. Some gcc versions/arches
(e.g. GCC 4.8 for x86_64) support -mpreferred-stack-boundary, but
only between 4 and 12. The message that -mpreferred-stack-boundary
wasn't supported was a little confusing. So explicitly say we checked
for -mpreferred-stack-boundary=2.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14324