build: Copy and use Clang's vector intrinsics headers.

When compiling with GCC, these headers get pulled in from the
gcc_syslibs_devel package, but we cannot do something similar
for Clang as Clang adds/removes internal builtins used by
the headers nearly every version. So instead we just copy
all the intrinsics headers from current Clang into generated,
and make sure this directory is included before any others.

build/jam/HeadersRules

@@ -456,6 +456,10 @@ rule FStandardHeaders architecture
 
 	local headers = ;
 
+	if $(HAIKU_CC_IS_CLANG_$(architecture)) = 1 {
+		headers += [ FDirName $(HAIKU_OUTPUT_DIR) clang_headers ] ;
+	}
+
 	# The c++ headers.
 	headers += [ C++HeaderDirectories $(architecture) ] ;