Several tests with `//@ needs-force-clang-based-tests` don't seem to run anywhere

[Zalathar]

(Follow-up to #126180 and #126257.)

There are currently 5 tests that use the needs-force-clang-based-tests header (formerly needs-matching-clang):

• tests/run-make/cross-lang-lto-clang/Makefile (this one does actually run)
• tests/run-make/cross-lang-lto-pgo-smoketest/Makefile
• tests/run-make/cross-lang-lto-riscv-abi/rmake.rs
• tests/run-make/issue-84395-lto-embed-bitcode/Makefile
• tests/run-make/wasm-override-linker/Makefile

Tests with this header only run when RUSTBUILD_FORCE_CLANG_BASED_TESTS is set. That environment variable is only set in the x86_64-gnu-debug job, and only when running x --stage 2 test tests/run-make --test-args clang. So in practice, 4/5 of the above tests don't appear to ever run in any CI job.

@rustbot label +A-testsuite

[Kobzol]

For completeness, this is the full list of run-make tests that are currently not being executed in any CI job, according to my CI scripts:

[run-make] tests/run-make/cross-lang-lto-pgo-smoketest
[run-make] tests/run-make/cross-lang-lto-riscv-abi
[run-make] tests/run-make/issue-84395-lto-embed-bitcode
[run-make] tests/run-make/wasm-exceptions-nostd
[run-make] tests/run-make/wasm-override-linker
[run-make] tests/run-make/windows-safeseh
[run-make] tests/run-make/x86_64-fortanix-unknown-sgx-lvi