cmake: Check for zephyr-gnu when force -O2 for C++ exceptions

If the application is using exceptions, we need to use
SPEED_OPTIMIZATIONS to get the libstdc++ version with exception
support.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

cmake/zephyr/Kconfig

@@ -25,5 +25,5 @@ config PICOLIBC_DEFAULT
 	  Zephyr SDK >=0.17.1 always uses Picolibc
 
 choice COMPILER_OPTIMIZATIONS
-	default SPEED_OPTIMIZATIONS if "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr" && CPP_EXCEPTIONS
+	default SPEED_OPTIMIZATIONS if ("$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr" || "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr-gnu") && CPP_EXCEPTIONS
 endchoice