lib/libc: Add FULL_LIBC_SUPPORTED helper Kconfig symbol

This symbol is selected when the target has any full libc available. This
allows tests to filter on this condition. It doesn't depend on whether the
application actually selects that library, only whether requesting a full C
library would work.

Signed-off-by: Keith Packard <keithp@keithp.com>
diff --git a/Kconfig.zephyr b/Kconfig.zephyr
index 827b19a..225d947 100644
--- a/Kconfig.zephyr
+++ b/Kconfig.zephyr
@@ -316,6 +316,7 @@
 
 config NATIVE_APPLICATION
 	bool "Build as a native host application"
+	select FULL_LIBC_SUPPORTED
 	help
 	  Build as a native application that can run on the host and using
 	  resources and libraries provided by the host.