[cmake] Some command line arguments not persisting to stage0 step

Prerequisites

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Description

Some variables passed to cmake (such as CMAKE_C_COMPILER) at configuration step do not persist when the project is built.

Context

The first thing the CMakeLists.txt file does is iterate through all cached cmake variables and set some special variables that are passed on to step0. But when the project is created with the project command, some of these variables are set to default values.
Additionally, custom toolchain files are invalidated because they are not read until the project is created.

Steps to Reproduce

In a system with both clang and gcc installed, with gcc as the default compiler:

1. cmake -B build -DCMAKE_C_COMPILER=clang ==> The C compiler identification is Clang 17.0.4
2. make -C build ==> -- The C compiler identification is GNU 13.2.0

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I noticed other cmake-related issues could be directly related to this.
Would a PR to attempt to fix this be acceptable?

Start by reproducing the issue with cmake -B build -DCMAKE_C_COMPILER=clang followed by make -C build, then inspect the variable iteration and project handling in CMakeLists.txt. Done means the selected compiler and custom toolchain settings persist into the stage0 build, with the reported reproduction behaving consistently.