gcc: default to 15.2.0

target/config/Config.in.compiler

@@ -22,7 +22,7 @@ default ADK_TOOLCHAIN_GCC_NDS32 if ADK_TARGET_ARCH_NDS32
 default ADK_TOOLCHAIN_GCC_7 if ADK_TARGET_ARCH_C6X # ICE with newer GCC
 default ADK_TOOLCHAIN_GCC_10 if ADK_TARGET_ARCH_CRIS # no longer supported
 default ADK_TOOLCHAIN_GCC_10 if ADK_TARGET_CPU_SH_J2 # j2.patch
-default ADK_TOOLCHAIN_GCC_14
+default ADK_TOOLCHAIN_GCC_15
 
 config ADK_TOOLCHAIN_GCC_GIT
 	bool "git"