gcc: default to gcc8

target/config/Config.in.compiler

@@ -35,7 +35,7 @@ default ADK_TOOLCHAIN_GCC_CSKY if ADK_TARGET_ARCH_CSKY
 default ADK_TOOLCHAIN_GCC_METAG if ADK_TARGET_ARCH_METAG
 default ADK_TOOLCHAIN_GCC_NDS32 if ADK_TARGET_ARCH_NDS32
 default ADK_TOOLCHAIN_GCC_OR1K if ADK_TARGET_ARCH_OR1K
-default ADK_TOOLCHAIN_GCC_7
+default ADK_TOOLCHAIN_GCC_8
 
 config ADK_TOOLCHAIN_GCC_GIT
 	bool "git"