gsdx: try to ask GCC to generate not dumb code

Unfortunately it requires at least GCC6. If a nice guy can check the generated code on GCC6.
I don't know clang status.

Here the only example, I have found on the web
https://developers.redhat.com/blog/2016/02/25/new-asm-flags-feature-for-x86-in-gcc-6/

Current generated code in GSTextureCache::SourceMap::Add

    38b3:	bsf    eax,esi
    38b6:	add    esp,0x10
    38b9:	test   esi,esi
    38bb:	jne    387e <GSTextureCache::SourceMap::Add(GSTextureCache::Source*, GIFRegTEX0 const&, GSOffset*)+0x6e>

BSF already set the Z flag when input (esi) is 0. So it would be better
to not put a silly add before the jump and to skip the test operation.

Author: gregory38

plugins/GSdx/stdafx.h

@@ -391,11 +391,17 @@ using namespace stdext;
 
 	// http://svn.reactos.org/svn/reactos/trunk/reactos/include/crt/mingw32/intrin_x86.h?view=markup
 
-	__forceinline unsigned char _BitScanForward(unsigned long* const Index, const unsigned long Mask)
+	__forceinline int _BitScanForward(unsigned long* const Index, const unsigned long Mask)
 	{
-		__asm__("bsfl %k[Mask], %k[Index]" : [Index] "=r" (*Index) : [Mask] "mr" (Mask));
-		
+#if defined(__GCC_ASM_FLAG_OUTPUTS__) && 0
+		// Need GCC6 to test the code validity
+		int flag;
+		__asm__("bsfl %k[Mask], %k[Index]" : [Index] "=r" (*Index), "=@ccz" (flag) : [Mask] "mr" (Mask));
+		return flag;
+#else
+		__asm__("bsfl %k[Mask], %k[Index]" : [Index] "=r" (*Index) : [Mask] "mr" (Mask) : "cc");
 		return Mask ? 1 : 0;
+#endif
 	}
 
 	#ifdef __GNUC__