Port WAMR to the FreeBSD platform and update the document (#1825)

product-mini/platforms/freebsd/build_jit.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+rm -fr build && mkdir build
+cd build
+cmake .. -DWAMR_BUILD_JIT=1
+nproc=$(sysctl -n hw.ncpu)
+make -j ${nproc}
+cd ..