samples: pidfd: build sample program for target architecture

This userspace program includes UAPI headers exported to usr/include/.
'make headers' always works for the target architecture (i.e. the same
architecture as the kernel), so the sample program should be built for
the target as well. Kbuild now supports 'userprogs' for that.

I also guarded the CONFIG option 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

@@ -122,7 +122,7 @@ config SAMPLE_HIDRAW
 
 config SAMPLE_PIDFD
 	bool "pidfd sample"
-	depends on HEADERS_INSTALL
+	depends on CC_CAN_LINK && HEADERS_INSTALL
 
 config SAMPLE_SECCOMP
 	bool "Build seccomp sample code"

samples/pidfd/Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 
-hostprogs := pidfd-metadata
-always-y := $(hostprogs)
-HOSTCFLAGS_pidfd-metadata.o += -I$(objtree)/usr/include
-all: pidfd-metadata
+usertprogs := pidfd-metadata
+always-y := $(userprogs)
+
+userccflags += -I usr/include