gcc: nios2 is no longer supported in gcc 15.x

target/config/Config.in.compiler

@@ -22,6 +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 if ADK_TARGET_ARCH_NIOS2 # no longer supported
 default ADK_TOOLCHAIN_GCC_15
 
 config ADK_TOOLCHAIN_GCC_GIT