Fix wamr compiler issues and refine some error messages (#470)

Fix potential memory leak issue when using llvm::EngineBuilder().selectTarget()
Fix issue of accessing aot_value's fields after it is freed
Fix JIT not print failed to link import warning
Change some error messages: 'fail to' to 'failed to'
Update error message when SIMD isn't enabled
Fix install littlevgl wasm app of wasi version failed

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

core/iwasm/compilation/aot_compiler.c

@@ -1036,8 +1036,8 @@ build_atomic_rmw:
       case WASM_OP_SIMD_PREFIX:
       {
         if (!comp_ctx->enable_simd) {
-            aot_set_last_error(
-              "current building does not support SIMD instructions");
+            aot_set_last_error("SIMD instruction was found, "
+                               "try adding --enable-simd option?");
             return false;
         }