lib/libc: newlib use depends upon newlib being supported

Before allowing newlib to be selected as the C library, ensure that it is
available for the target.

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

Author: keith-packard

lib/libc/Kconfig

@@ -77,6 +77,7 @@ config PICOLIBC
 config NEWLIB_LIBC
 	bool "Newlib C library"
 	depends on !NATIVE_APPLICATION
+	depends on NEWLIB_LIBC_SUPPORTED
 	help
 	  Build with newlib library. The newlib library is expected to be
 	  part of the SDK in this case.