cmake: gnu: Set TOOLCHAIN_HAS_GLIBCXX

This commit updates the GNU toolchain CMake script to set
`TOOLCHAIN_HAS_GLIBCXX` to indicate that libstdc++ aka. GNU C++ Standard
Library is supported.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

cmake/zephyr/gnu/generic.cmake

@@ -29,5 +29,7 @@ endif()
 
 set(CROSS_COMPILE ${one_toolchain_root}/${CROSS_COMPILE_TARGET}/bin/${CROSS_COMPILE_TARGET}-)
 set(SYSROOT_DIR   ${one_toolchain_root}/${SYSROOT_TARGET}/${SYSROOT_TARGET})
+
 set(TOOLCHAIN_HAS_NEWLIB ON CACHE BOOL "True if toolchain supports newlib")
 set(TOOLCHAIN_HAS_PICOLIBC ON CACHE BOOL "True if toolchain supports picolibc")
+set(TOOLCHAIN_HAS_GLIBCXX ON CACHE BOOL "True if toolchain supports libstdc++")