CoreFoundation: add a workaround for Linux with the rebranch

This adds an optimization barrier in the loop that is potentially
getting mis-optimized with the rebranch on Linux.  This should allow us
to rebranch and follow up with a proper fix subsequently.

Thanks to @etcwilde for tracking down this issue!

Author: compnerd

CoreFoundation/String.subproj/CFAttributedString.c

@@ -350,6 +350,9 @@ CFDictionaryRef CFAttributedStringGetAttributesAndLongestEffectiveRange(CFAttrib
 	// Now go back and set leftEnd
 	tmpRange = *longestEffectiveRange;
         do {
+#if defined(__linux__)
+            __asm__ __volatile__("" : : : "memory");
+#endif
 	    if ((leftEnd = tmpRange.location) <= rangeLimit.location) break;
             other = CFAttributedStringGetAttributes(attrStr, leftEnd - 1, &tmpRange);
         } while (CFEqual(other, attrs));