update gcc version default to 6.3.0

Makefile

@@ -4,7 +4,7 @@ SOURCES = sources
 
 CONFIG_SUB_REV = 3d5db9ebe860
 BINUTILS_VER = 2.27
-GCC_VER = 6.2.0
+GCC_VER = 6.3.0
 MUSL_VER = 1.1.16
 GMP_VER = 6.1.1
 MPC_VER = 1.0.3