[PowerPC] Temporarily disable VSX for PowerPC fast-isel tests

Patch by Bill Seurer; some comment formatting changes by me.

There are a few PowerPC test cases for FastISel support that currently
fail with VSX support enabled.  The temporary workaround under
discussion in http://reviews.llvm.org/D5362 helps, but the tests still
fail because they specify -fast-isel-abort, and the VSX workaround
punts back to SelectionDAG.  We have plans to fix FastISel permanently
for VSX, but until that's in place these tests are preventing us from
enabling VSX by default.  Therefore we are adding -mattr=-vsx to these
tests until the full support is ready.