cmake: Disable Newlib-nano for Xtensa

This commit temporarily disables the Newlib nano variant for the Xtensa
architecture because there is a bug in the nano `memset` implementation
causing a system crash (see the GitHub issue zephyrproject-rtos#660).

Revert this commit when zephyrproject-rtos#660 is fixed.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

cmake/zephyr/Kconfig

@@ -2,7 +2,9 @@
 
 config TOOLCHAIN_ZEPHYR_0_16
 	def_bool y
-	select HAS_NEWLIB_LIBC_NANO
+	# FIXME: Newlib-nano is disabled for Xtensa targets due to the memset
+        #        bug causing crashes (see the GitHub issue #660).
+	select HAS_NEWLIB_LIBC_NANO if !XTENSA
 
 config TOOLCHAIN_ZEPHYR_SUPPORTS_THREAD_LOCAL_STORAGE
 	def_bool y