Simply disable the broken path rather than worry about it for now.

Likely an easy fix but would rather this be handled upstream and
reported by someone that is affected and can test.

https://bugs.gentoo.org/926287
--- a/src/gui/painting/qrgbafloat.h
+++ b/src/gui/painting/qrgbafloat.h
@@ -20,5 +20,5 @@
 public:
     using Type = F;
-#if defined(__AVX512FP16__) && QFLOAT16_IS_NATIVE
+#if 0
     // AVX512FP16 has multiplication instructions
     using FastType = F;