build: add initial NetBSD support.

Not able to generate ELF files on NetBSD yet (lacks the note and crt1.o
is actually named crt0.o on NetBSD), but -run works with these extra
defines:
-D__lint__ -D"__symbolrename(x)=asm(#x)" -D__NetBSD__

The -D__lint__ is an ugly hack, TCC should be able to emulate GCC just
fine, but it seems TCC doesn't support __builtin_va_list yet?
	typedef __builtin_va_list __va_list;
/usr/include/sys/ansi.h:72: error: ';' expected (got "__va_list")

configure

@@ -56,6 +56,7 @@ case $targetos in
   DragonFly) noldl=yes;;
   OpenBSD)   noldl=yes;;
   FreeBSD)   noldl=yes;;
+  NetBSD)    noldl=yes;;
   *) ;;
 esac