Support integrate 3rd-party toolchains into wamrc (#1237)

Support integrating 3rd-party toolchain llc compiler or asm compiler
into wamrc by setting environment variable WAMRC_LLC_COMPILER
or WAMRC_ASM_COMPILER, wamrc will use these tools to generate
object file from LLVM IR firstly, and then refactor the object file into
aot file.

core/iwasm/compilation/aot_compiler.h

@@ -371,6 +371,10 @@ aot_emit_aot_file_buf(AOTCompContext *comp_ctx, AOTCompData *comp_data,
 bool
 aot_emit_object_file(AOTCompContext *comp_ctx, char *file_name);
 
+char *
+aot_generate_tempfile_name(const char *prefix, const char *extension,
+                           char *buffer, uint32 len);
+
 #ifdef __cplusplus
 } /* end of extern "C" */
 #endif