Add support for the x86intrin.h header on GCC systems.

This header can be included at any time on x86 systems and is present
since the GCC versiosn that also support AVX.  It contains intrinsics
for instructions that have been present in x86 CPUs since the dawn of
time.

Change-Id: I9adb066c2c0b56ce8fd5ed7366716038f1254502
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

src/corelib/tools/qsimd_p.h

@@ -139,6 +139,12 @@ QT_BEGIN_HEADER
 #  endif
 #endif
 
+// other x86 intrinsics
+#if defined(QT_COMPILER_SUPPORTS_AVX) && defined(Q_CC_GNU)
+#define QT_COMPILER_SUPPORTS_X86INTRIN
+#include <x86intrin.h>
+#endif
+
 // NEON intrinsics
 #if defined __ARM_NEON__
 #include <arm_neon.h>