Fix some compile issues of samples (#690)

Fix compile errors of workloads due to emsdk version upgrade,
enable BUILD_TARGET auto set for some samples,
and call wasm_runtime_init_thread_env() for in thread routine which
was created by pthread_create but not runtime in spawn-thread sample.

Signed-off-by: Wenyong Huang <wenyong.huang@intel.com>

samples/workload/tensorflow/README.md

@@ -5,8 +5,8 @@ This sample demonstrates how to build [tensorflow](https://github.com/tensorflow
 ```bash
 git clone https://github.com/emscripten-core/emsdk.git
 cd emsdk
-./emsdk install latest
-./emsdk activate latest
+./emsdk install 2.0.12
+./emsdk activate 2.0.12
 ```
 And set up ensdk environment:
 ```bash
@@ -26,4 +26,4 @@ to build tensorflow and run it with iwasm, which basically contains the followin
 - build tf-lite with emcc compiler
 - build iwasm with pthread enable and include libiary under libc-emcc
 - run benchmark model with iwasm:
-  --max-secs 300: means the max training time cost is 5 minutes, you can adjust by yourself
+  --max-secs 300: means the max training time cost is 5 minutes, you can adjust it by yourself

samples/workload/tensorflow/build.sh

@@ -17,7 +17,7 @@ fi
 
 set -xe
 
-EMSDK_WASM_DIR="$EM_CACHE/sysroot/lib/wasm32-emscripten"
+EMSDK_WASM_DIR="${EMSDK}/upstream/emscripten/cache/wasm"
 BUILD_SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 OUT_DIR="${BUILD_SCRIPT_DIR}/out"
 TENSORFLOW_DIR="${BUILD_SCRIPT_DIR}/tensorflow"