sparc: default to gcc 7.x, newer gcc breaks bootup

target/config/Config.in.compiler

@@ -20,6 +20,7 @@ default ADK_TOOLCHAIN_GCC_METAG if ADK_TARGET_ARCH_METAG
 default ADK_TOOLCHAIN_GCC_NDS32 if ADK_TARGET_ARCH_NDS32
 default ADK_TOOLCHAIN_GCC_7 if ADK_TARGET_CPU_ARM_ARM11MPCORE
 default ADK_TOOLCHAIN_GCC_7 if ADK_TARGET_CPU_ARM_CORTEX_A53
+default ADK_TOOLCHAIN_GCC_7 if ADK_TARGET_ARCH_SPARC
 default ADK_TOOLCHAIN_GCC_9 if ADK_TARGET_ARCH_CSKY
 default ADK_TOOLCHAIN_GCC_9 if ADK_TARGET_ARCH_HPPA
 default ADK_TOOLCHAIN_GCC_9 if ADK_TARGET_ARCH_OR1K