cmake: Make Zephyr use full libstdc++ with CPP_EXCEPTIONS

When we're using C++ exceptions, we need to use the full version of
libstdc++ that includes exception support. With multilib-space, that
means using the -O2 version instead of the -Os version.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

cmake/zephyr/Kconfig

@@ -22,3 +22,7 @@ config PICOLIBC_DEFAULT
 	depends on "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr"
 	help
 	  Zephyr SDK >=0.17.1 always uses Picolibc
+
+choice COMPILER_OPTIMIZATIONS
+	default SPEED_OPTIMIZATIONS if "$(ZEPHYR_TOOLCHAIN_VARIANT)" = "zephyr" && CPP_EXCEPTIONS
+endchoice