ArchitectureRules: Disable the 'fork' builtin on GCC4+.

On GCC 7.3, it conflicts with our definition of 'fork'. The documentation
states that disabling builtins has no effect on versions where they
do not exist, so we don't need to check for GCC7 here.

build/jam/ArchitectureRules

@@ -28,7 +28,7 @@ rule ArchitectureSetup architecture
 	#		our gcc4 compiler. See this discussion on some issues:
 	# http://www.freelists.org/post/haiku-development/hrev45320-Yet-another-nonobvious-effect-of-ftreevrp-optimization
 	if $(gccVersion[1]) >= 3 {
-		gccBaseFlags += -fno-strict-aliasing ;
+		gccBaseFlags += -fno-strict-aliasing -fno-builtin-fork ;
 		if $(HAIKU_CC_IS_CLANG_$(architecture)) != 1 {
 			gccBaseFlags += -fno-tree-vrp ;
 		}