Use target config for toolchain

[eagleonhill]

Toolchain typically contains two parts: Compiler and runtime.
Runtime should be using target config.
For compilers that needs host/exec config, the toolchain should do own
transition.

Currently the toolchain is using host config if enabled new toolchain
resolution, which caused the runtime not able to be linked as expected.

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[katre]

This is a known issue that has been reported several times (see #4067 and #6889). Unfortunately, simply using the target configuration is also not always correct: consider the case of a toolchain that wants to build the compiler itself (which would need to use the exec configuration).

I have proposed a solution called Toolchain Transitions which allows for both cases, and I have opened an issue tracking that work (#10523).

I am closing this PR but please read the proposal and leave any comments on #10523 if there is a case that isn't covered there.