[RFC] XIP support of AOT mode

[no1wudi]

Now WAMR apply symbol relocation at run time, that must load AOT code into ram.

This operation can be done at compile time, user can provide a address to let AOT compiler know the final symbol address. Then the AOT module can be written to flash and execute in place.

By this, we can save ram space and get better performance (no symbol relocation at runtime).

Then we need:

• A unified relocator
• A new mode flag
• A method to resolve symbol address at compile time
• Enhancement of wamrc

unified relocator

Now build system choose one implemtion of relocator by compile target setting :

We need let all of them avaliable in wamrc, then the relocation function need to have a unique symbol name or define a relocator like this:

struct relocator {
    apply_reloc(...)
    ***
}

And each implemention return a relocator reference to AOT module or wamrc.

new mode flag

May we can provide a new mode like Wasm_Module_AoT_XIP and corresponding compile option, the we can disable relocator on runtime side.

enhancement of wamrc

Now wamrc use LLVM to generate target code, and then need some minor change to support run relocate operation after compile (be careful of target byte endian).

[no1wudi]

And the most difficult thing is: how to resolve symbol address at compile time.

In AOT mode, native code was emitted in LLVMRelocStatic mode, all the function call need to use relative address.

WAMR runtime will load all AOT code into RAM, and do relocation at run time.Because only in the real execution environment, we can get the exact address of these symbols.

During compilation, we do not know where these system functions will be placed in the execution environment.

Now we have the following options：

1. The simplest way: before executing, relocate the module and write it to flash, and then execute
2. First export the required symbol table from the target system, and then complete the relocation on the host
3. In the AOT module, each function call is added with additional execution environment information, and the symbol table is placed in it. All function calls are made indirectly through this symbol table, similar to aot_invoke_native.