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")

lib/bcheck.c

@@ -22,7 +22,7 @@
 #include <stdarg.h>
 #include <string.h>
 #if !defined(__FreeBSD__) && !defined(__FreeBSD_kernel__) \
-    && !defined(__DragonFly__) && !defined(__OpenBSD__)
+    && !defined(__DragonFly__) && !defined(__OpenBSD__) && !defined(__NetBSD__)
 #include <malloc.h>
 #endif
 #if !defined(_WIN32)
@@ -41,7 +41,7 @@
 
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__) \
     || defined(__DragonFly__) || defined(__dietlibc__) \
-    || defined(__UCLIBC__) || defined(__OpenBSD__) \
+    || defined(__UCLIBC__) || defined(__OpenBSD__) || defined(__NetBSD__) \
     || defined(_WIN32) || defined(TCC_UCLIBC)
 #warning Bound checking does not support malloc (etc.) in this environment.
 #undef CONFIG_TCC_MALLOC_HOOKS