arc64: Note that gdb is not supported

Note that gdb is currently not supported for the ARC64 target and we
are intentionally not building it.

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

Author: stephanosio

configs/arc64.config

@@ -35,6 +35,7 @@ CT_GCC_DEVEL_URL="https://github.com/zephyrproject-rtos/gcc.git"
 CT_GCC_DEVEL_REVISION="2e88258647d565da6aa82470a8f39cf97e09a4a8"
 CT_CC_GCC_EXTRA_CONFIG_ARRAY="--with-gnu-ld --with-gnu-as --enable-initfini-array"
 CT_CC_LANG_CXX=y
+# CT_DEBUG_GDB is not set
 CT_COMP_LIBS_NEWLIB_NANO=y
 CT_NEWLIB_NANO_SRC_DEVEL=y
 CT_NEWLIB_NANO_DEVEL_URL="https://github.com/zephyrproject-rtos/newlib-cygwin.git"