bump default gcc version to 9.2.0

Makefile

@@ -3,7 +3,7 @@ SOURCES = sources
 
 CONFIG_SUB_REV = 3d5db9ebe860
 BINUTILS_VER = 2.33.1
-GCC_VER = 8.3.0
+GCC_VER = 9.2.0
 MUSL_VER = 1.1.24
 GMP_VER = 6.1.2
 MPC_VER = 1.1.0