tools/nolibc: make sys_mmap() automatically use the right __NR_mmap definition

__NR_mmap2 was used for i386 but it's also needed for other archs such
as RISCV32 or ARM. Let's decide to use it based on the __NR_mmap2
definition as it's not defined on other archs.
Signed-off-by: NWilly Tarreau <w@1wt.eu>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/sys.h

@@ -697,7 +697,7 @@ void *sys_mmap(void *addr, size_t length, int prot, int flags, int fd,
 
 	int n;
 
-#if defined(__i386__)
+#if defined(__NR_mmap2)
 	n = __NR_mmap2;
 	offset >>= 12;
 #else