gcc: disable libitm in toolchain stage

Note:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51072

base/gcc/gcc.conf

@@ -154,6 +154,7 @@ if atstage toolchain; then
 	var_append confopt " " "--disable-libssp"
 	var_append confopt " " "--disable-libgomp"
 	var_append confopt " " "--disable-libquadmath"
+	var_append confopt " " "--disable-libitm"
 
 	# use the native libraries we build in the toolchain directory
 	export LDFLAGS="-Wl,-rpath,${root}${libdir}"