[x86] Change the test added in r223774 to first check the spelling of
the error message for a bogus processor, and then look specifically for
that error message using FileCheck.

I actually tried to write the test this way at first, but drew a blank
on how to ensure the error message stayed in sync (oops). Now that I've
recalled how to do that, this is clearly better.

It also fixes an issue with a malloc implementation that actually prints
to stderr in all cases, which was causing problems for some builders it
seems.