x86: Disable AVX support on 64-bit MinGW

GCC for 64-bit Windows has a bug that it fails to properly re-align the
stack pointer for use with 256-bit memory addresses (AVX). Therefore,
there's about a 50/50 chance that any function using AVX will have an
improperly-aligned stack. In release mode, stack accesses should be
rare, but in debug mode they happen frequently. Either way, this is a
ticking time bomb, so we disable.

Clang is not affected.
32-bit MinGW is not affected.
64-bit in other OSes with GCC are not affected.

Fixes: QTBUG-73539
Change-Id: Id061f35c088044b69a15fffd1580967808f31671
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

config.tests/x86_simd/main.cpp

@@ -144,6 +144,9 @@ attribute_target("sha") void test_shani()
 #endif
 
 #if T(AVX)
+#  if defined(__WIN64__) && defined(__GNUC__) && !defined(__clang__)
+#    error "AVX support is broken in 64-bit MinGW - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49001"
+#  endif
 attribute_target("avx") void test_avx()
 {
     __m256d a = _mm256_setzero_pd();