Prevent run-make test recipes (rmake.rs) from using *any* unstable features #137532

Three rmake.rs use unstable features that cannot be used (because they need to be build by the stage0 compiler which usually is not a nightly compiler), and needs to be fixed (this was a bug in the previous infra implementation and partially rectified in #137373):

• tests/run-make/issue-107495-archive-permissions/rmake.rs:

  rust/tests/run-make/issue-107495-archive-permissions/rmake.rs

  Line 1 in f43e549

  #![feature(rustc_private)]

• tests/run-make/cross-lang-lto/rmake.rs:

  rust/tests/run-make/cross-lang-lto/rmake.rs

  Line 6 in f43e549

  #![feature(path_file_prefix)]

• tests/run-make/broken-pipe-no-ice/rmake.rs:

  rust/tests/run-make/broken-pipe-no-ice/rmake.rs

  Line 14 in f43e549

  #![feature(anonymous_pipe)]

After these tests are fixed and #137373 merges, we need to:

• Build rmake.rs with RUSTC_BOOTSTRAP=-1 to prevent any nightly feature usages within rmake.rs recipes by convincing the stage 0 compiler used to build rmake.rs that it is a stable-channel compiler. Prevent rmake.rs from using unstable features, and fix 3 run-make tests that currently do #137537
• Explicitly document this requirement in rustc-dev-guide. Prevent rmake.rs from using unstable features, and fix 3 run-make tests that currently do #137537

FIXME

• Replace os_pipe with anonymous_pipe once it stabilizes and reaches beta. cf. Stabilize anonymous_pipe #135822 or Stablize anonymous pipe #137793.
  • run-make: drop os_pipe workaround now that anonymous_pipe is stable on beta #139976