Rollup merge of #127958 - jieyouxu:compiletest-rmake-cleanup, r=Kobzol

Cleanup rmake.rs setup in compiletest

While debugging rmake.rs tests I realized that the rmake.rs setup itself in compiletest is very messy and confused. Now that I know some of the bootstrap steps and the rmake.rs tests themselves better, I realized there are cleanups that are possible:

- Rework how `source_root` and `build_root` are calculated. They should now be less fragile then before.
- Shuffle around path calculations to make them more logically grouped and closer to eventual use site(s).
- Cleanup executable extension calculation with `std::env::consts::EXE_EXTENSION`.
- Cleanup various dylib search path handling: renamed variables to better reflect their purpose, minimized mutability scope of said variables.
- Prune useless env vars passed to both `rustc` and recipe binary commands.
- Vastly improve the documentation for the setup of rmake.rs tests, including assumed bootstrap-provided build layouts, rmake.rs test layout, dylib search paths, intended purpose of passed env vars and the `COMPILETEST_FORCE_STAGE0=1 ./x test run-make --stage 0` stage0 sysroot special handling.

This PR is best reviewed commit-by-commit.

Fixes https://github.com/rust-lang/rust/issues/127920.

r? bootstrap (or Kobzol, or Mark, or T-compiler)

try-job: aarch64-apple
try-job: armhf-gnu
try-job: dist-x86_64-linux
try-job: test-various
try-job: x86_64-mingw
try-job: x86_64-msvc
try-job: x86_64-gnu-llvm-18

0 files changed, 0 insertions, 0 deletions