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.h

@@ -15,7 +15,9 @@
 extern "C" {
 #endif
 
+#ifndef AOT_FUNC_PREFIX
 #define AOT_FUNC_PREFIX "aot_func#"
+#endif
 
 typedef InitializerExpression AOTInitExpr;
 typedef WASMType AOTFuncType;