Add support for OpenBSD

It just rejects the generic System V ABI and has other syscall numbers.

Makefile

@@ -1,11 +1,13 @@
 # All we need is C99 and POSIX, which this should make available
 CFLAGS = -std=gnu99
-NAMES = bfc-amd64-linux
+NAMES = bfc-amd64-linux bfc-amd64-openbsd
 
 all: $(NAMES)
 
-%: %.c
-	$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $@
+%-linux: %.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $@ -DTARGET_LINUX
+%-openbsd: %.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $@ -DTARGET_OPENBSD
 clean:
 	rm -f $(NAMES)