dev-qt/qtbase: workaround x86intrin test and feature selection

qtbase does a single "big" test for all basic cpu features, and if
CXXFLAGS have any matching -mno-* then they will override Qt's
flags and the test fails. Surprised by this, Qt aborts unless
explicitly disable the feature.

Test can be bypassed, but then it does not perform per-flags
tests and will fail to build as-is.

Like bug #913400, debated a few options:

1. cpu_flags_x86_* to enable each feature, but given this is tied to
   compiler flags we'd still need to test them to avoid failures and
   not much better off, not to mention bug #913400 may have added its
   own -mno-* that go against these.

2. Patching cmake files so it always pass its e.g. -mavx2 after the
   user's flags (when needed), and does not rely on -march=haswell
   given that does not override. But fwiw these files do get
   installed and will alter expected behavior, and handling
   -march is more annoying.

3. Patch out the bit that makes the x86intrin test prevent building
   unless explicitly disabled, and let it auto-disable x86intrin
   entirely if tests fail (subpar for performance if ignored).

4. Do self-tests and disable features that will fail, this has the
   advantage that revdeps will not try to use these either.

5. One option to bug #913400 was to force simpler flags, which
   would also solve this.

Picked #4 for now, not that particularly like it given it feels
like automagic. Hoping will be more temporary than qsimd_p.h
workarounds. Better than doing nothing either way, is a no-op
for non-affected users.

Bug: https://bugs.gentoo.org/908420
Closes: https://bugs.gentoo.org/913400
Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

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