samples: auxdisplay: 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: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>

samples/auxdisplay/Makefile

@@ -1,10 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
-CC := $(CROSS_COMPILE)gcc
-CFLAGS := -I../../usr/include
-
-PROGS := cfag12864b-example
-
-all: $(PROGS)
-
-clean:
-	rm -fr $(PROGS)
+userprogs := cfag12864b-example
+always-y := $(userprogs)