selftests/nolibc: integrate with kselftests

Hook up nolibc-test with the kselftests framework.
This enables CI systems and developers to easily execute the tests.

While nolibc-test does not emit KTAP output itself that is not a problem,
as the kselftest executor will wrap the output in KTAP.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250620-nolibc-selftests-v1-4-f6b2ce7c5071@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/testing/selftests/Makefile

@@ -77,6 +77,7 @@ TARGETS += net/ovpn
 TARGETS += net/packetdrill
 TARGETS += net/rds
 TARGETS += net/tcp_ao
+TARGETS += nolibc
 TARGETS += nsfs
 TARGETS += pci_endpoint
 TARGETS += pcie_bwctrl

tools/testing/selftests/nolibc/Makefile

@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: GPL-2.0
+
+TEST_GEN_PROGS := nolibc-test
+
+include ../lib.mk
+include $(top_srcdir)/scripts/Makefile.compiler
+
+cc-option = $(call __cc-option, $(CC),,$(1),$(2))
+
+include Makefile.include
+
+CFLAGS = -nostdlib -nostdinc -static \
+	 -isystem $(top_srcdir)/tools/include/nolibc -isystem $(top_srcdir)/usr/include \
+	 $(CFLAGS_NOLIBC_TEST)
+
+ifeq ($(LLVM),)
+LDLIBS := -lgcc
+endif
+
+$(OUTPUT)/nolibc-test: nolibc-test.c nolibc-test-linkage.c | headers
+
+help:
+	@echo "For the custom nolibc testsuite use '$(MAKE) -f Makefile.nolibc'; available targets:"
+	@$(MAKE) -f Makefile.nolibc help
+
+.PHONY: help