Support thunking of multi-instance Vulkan applications

FEX's libvulkan thunks don't work if multiple `VkInstances` are used that return different function pointers to `vkGetDeviceProcAddr`. This is because to implement `vkGetDeviceProcAddr` in the first place, FEX needs to pre-query a `VkInstance`-specific function pointer during initialization. The function pointer itself is called without any direct reference to the original `VkInstance`, so FEX acts as if only the first VkInstance created by the application was used.

In the broader picture, the problem is a restriction in the thunk generator: Functions that must be guest-callable through host function pointers can not use a custom guest entrypoint.

### Suggested solution: Extending framework for guest-callable host function pointers

Currently, guest-callable host function pointers are implemented by linking the host function pointer to a generic template function that forwards the packed argument list *plus* the original host function pointer to a `hostcall_` thunk. In this case, the function pointer is `vkGetDeviceProcAddr` as returned from `vkGetInstanceProcAddr`. What's needed is a way of customizing the target function: Instead of calling the thunk that initiates a Guest->Host transition, a custom function should be called.

Implementation sketch of the guest-side thunk library:
```cpp
template
inline Result ReadHiddenArgumentAndCall(Args... args) {
uintptr_t hidden_arg;
asm("mov %%rax, %0" : "=r" (hidden_arg));
return Function(args..., hidden_arg);
}

PFN_vkVoidFunction vkGetDeviceProcAddr_indirect(VkDevice a_0, const char* a_1, void* host_ptr) {
PackedArguments args = { a_0, a_1, host_ptr };
fexthunks_libvulkan_hostcall_vkGetDeviceProcAddr(&args);
LinkGuestAddressToHostFunction(args.rv, PtrsToLookUp.at(a_1));
return args.rv;
}

#if 0
// For illustration only: Public entrypoint of this function (if it were needed)
PFN_vkVoidFunction vkGetDeviceProcAddr(VkDevice a_0, const char* a_1) {
return vkGetDeviceProcAddr_indirect(a_0, a_1, fexldr_ptr_vkGetDeviceProcAddr);
}
#endif

PFN_vkVoidFunction vkGetInstanceProcAddr(VkInstance a_0, const char* a_1){
auto Ret = fexfn_pack_vkGetInstanceProcAddr(a_0, a_1);
if (a_1 != std::string_view { "vkGetDeviceProcAddr" }) {
LinkGuestAddressToHostFunction(Ret, PtrsToLookUp.at(a_1));
return Ret;
} else {
LinkGuestAddressToHostFunction(Ret, ReadHiddenArgumentAndCall);
return Ret;
}
}
```