samples: watchdog: use 'userprogs' syntax

Kbuild now supports the 'userprogs' syntax to compile userspace
programs for the same architecture as the kernel.

Add the entry to samples/Makefile to put this into the build bot
coverage.

I also added the CONFIG option guarded by 'depends on CC_CAN_LINK'
because $(CC) may not provide libc.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>

samples/Kconfig

@@ -205,5 +205,8 @@ config SAMPLE_INTEL_MEI
 	help
 	  Build a sample program to work with mei device.
 
+config SAMPLE_WATCHDOG
+	bool "watchdog sample"
+	depends on CC_CAN_LINK
 
 endif # SAMPLES

samples/Makefile

@@ -26,3 +26,4 @@ obj-$(CONFIG_VIDEO_PCI_SKELETON)	+= v4l/
 obj-y					+= vfio-mdev/
 subdir-$(CONFIG_SAMPLE_VFS)		+= vfs
 obj-$(CONFIG_SAMPLE_INTEL_MEI)		+= mei/
+subdir-$(CONFIG_SAMPLE_WATCHDOG)	+= watchdog

samples/watchdog/Makefile

 # SPDX-License-Identifier: GPL-2.0
-CC := $(CROSS_COMPILE)gcc
-PROGS := watchdog-simple
-
-all: $(PROGS)
-
-clean:
-	rm -fr $(PROGS)
-
+userprogs := watchdog-simple
+always-y := $(userprogs)