Update wasm app build scripts for wasi-sdk-12 and refine interpreter (#481)

Update wasm app build scripts for wasi-sdk-12.0: add --export=__main_argc_argv, remove --no-threads
Lookup function with name "__main_argc_argv" as main function besides "main"
Change module_malloc to runtime_malloc in wasi native lib
Refine classic interpreter op_block and op_br_table
Refine faster interpreter op_br_table

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

doc/build_wasm_app.md

@@ -64,18 +64,19 @@ There are some useful options which can be specified to build the source code:
 
 - **-Wl,--shared-memory** Use shared linear memory
 
-- **-Wl,--threads** or **-Wl,--no-threads** Run or do not run the linker multi-threaded
-
 - **-Wl,--allow-undefined** Allow undefined symbols in linked binary
 
 - **-Wl,--allow-undefined-file=<value>** Allow symbols listed in <file> to be undefined in linked binary
 
+- **-pthread** Support POSIX threads in generated code
+
 For example, we can build the wasm app with command:
 ``` Bash
 /opt/wasi-sdk/bin/clang -O3 -nostdlib \
     -z stack-size=8192 -Wl,--initial-memory=65536 \
-    -Wl,--export=main -o test.wasm test.c \
-    -Wl,--export=__heap_base,--export=__data_end \
+    -o test.wasm test.c \
+    -Wl,--export=main -Wl,--export=__main_argc_argv \
+    -Wl,--export=__heap_base -Wl,--export=__data_end \
     -Wl,--no-entry -Wl,--strip-all -Wl,--allow-undefined
 ```
 to generate a wasm binary with small footprint.