Test strconv::test::from_str_ignore_underscores fails on i686

[brson]

I'm going to xfail it.

http://buildbot.rust-lang.org/builders/incoming-full-mac/builds/1014/steps/test/logs/stdio

[bblum]

Tested on incoming with i686 cross-compile on x64. The test doesn't appear to be ignored, and seems to work fine.