Avoid __builtin_cpu_supports to workaround gcc bug. Refs #366

* Upstream stb_image.h uses __builtin_cpu_supports to detect if SSE2 is
  available, but GCC versions from at least 5.4 and up to apparently
  6.1.1 on Fedora 24 then give an error about a missing hidden symbol
  __cpu_model when the resulting code is compiled into a DSO.
* Avoiding this by just assuming x64 gives SSE2 will fix 99% of cases,
  and it's not a big loss for the tiny minority with SSE2 on x86 to miss
  out. Thanks to @Anteru for this suggested fix!

renderdoc/3rdparty/stb/stb_image.h

@@ -712,12 +712,15 @@ static int stbi__sse2_available()
 
 static int stbi__sse2_available()
 {
-#if defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__) >= 408 // GCC 4.8 or later
-   // GCC 4.8+ has a nice way to do this
-   return __builtin_cpu_supports("sse2");
+#if defined(STBI__X64_TARGET)
+   // on x64, SSE2 can be assumed to be available.
+   return 1;
 #else
-   // portable way to do this, preferably without using GCC inline ASM?
-   // just bail for now.
+   // __builtin_cpu_supports is buggy on GCC 5 and above, causing problems if
+   // referenced in a shared object, giving missing __cpu_model hidden symbol errors.
+   // To get around that, just assume that SSE2 is not available on x86.
+   //
+   // See https://github.com/nothings/stb/issues/280 for more information.
    return 0;
 #endif
 }