[cross-build] Simple C++/asm files are not built correctly

Hello,

I opened an issue on [Boost.Context](https://github.com/boostorg/context/issues/76) a while ago, and I'm currently trying to solve it "properly".

First, here is the small project I created to reproduce the issue:
[boost_build_bug.zip](https://github.com/boostorg/build/files/2565353/boost_build_bug.zip)

One of the problems is that the cross-compiling flags are not given to the compiler (I'm on macOS, and target iOS arm64/armv8):

```bash
$ b2 toolset=clang architecture=arm address-model=64 target-os=iphone asm -d+2
```

Here is the compiler invocation:

>"clang++" -x assembler-with-cpp -fPIC -O0 -fno-inline -Wall -g -c -o "bin/clang-darwin-10.0/debug/address-model-64/architecture-arm/target-os-iphone/make_arm64_aapcs_macho_gas.o" "make_arm64_aapcs_macho_gas.S"

I would expect at least the `-arch arm64` flag. Trying to build the `cpp` target is no different in this case (this behaviour looks global, not specific to assembly files).

I've read the user manual, and the [cross-building section](https://boostorg.github.io/build/manual/develop/index.html#bbv2.tasks.crosscompile) mentions the use of `user-config.jam`.
Is this the only way to support cross-building?

I do not mind writing a `user-config.jam`, but I find it confusing that there is no mapping between configuration flags (e.g. `architecture` + `address-model`) and compiler/linker flags (`-arch arm64` in this case).

I'm a Boost.Build novice though, so I'm likely to have missed the point.