rcutorture: Make use of nolibc when available

This reduces the size of the init executable from ~800 kB to ~800 bytes
on x86_64. This is only implemented for x86_64, i386, arm and arm64.
Others not tested.
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/bin/mkinitrd.sh

@@ -82,8 +82,10 @@ cd $D
 mkdir -p initrd
 cd initrd
 cat > init.c << '___EOF___'
+#ifndef NOLIBC
 #include <unistd.h>
 #include <sys/time.h>
+#endif
 
 volatile unsigned long delaycount;
 
@@ -113,7 +115,21 @@ int main(int argc, int argv[])
 	return 0;
 }
 ___EOF___
-${CROSS_COMPILE}gcc -s -static -Os -o init init.c
+
+# build using nolibc on supported archs (smaller executable) and fall
+# back to regular glibc on other ones.
+if echo -e "#if __x86_64__||__i386__||__i486__||__i586__||__i686__" \
+           "||__ARM_EABI__||__aarch64__\nyes\n#endif" \
+   | ${CROSS_COMPILE}gcc -E -nostdlib -xc - \
+   | grep -q '^yes'; then
+	# architecture supported by nolibc
+        ${CROSS_COMPILE}gcc -fno-asynchronous-unwind-tables -fno-ident \
+		-nostdlib -include ../bin/nolibc.h -lgcc -s -static -Os \
+		-o init init.c
+else
+	${CROSS_COMPILE}gcc -s -static -Os -o init init.c
+fi
+
 rm init.c
 echo "Done creating a statically linked C-language initrd"