Fix xpasses on the gcc buildbots using compiler versions to qualify the xfail.

- Note that this is not correct, as the failure is associated with build options of libc.so, however it's failing on a Debian buildbot that uses gcc 4.6.2 (and the real goal is a complete backtrace even with -fomit-frame-pointer).

- Adds helpers to lldbtest.py to check the expectedCompiler and expectedVersion, with an eventual goal of reducing the number of test decorators.
--- Currently allows a comparison operator and a compiler version to be specified.
--- Can be extended to support ranges of compiler versions.