Make --print=backend-has-zstd work by default on any backend

[Zalathar]

Using a defaulted CodegenBackend method that querying for zstd support should automatically print a safe value of false on any backend that doesn't specifically indicate the presence or absence of zstd.

This should fix the compiletest failures reported in #149666 (comment), which can occur when LLVM is not the default codegen backend.

[rustbot]

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[Zalathar]

cc @bjorn3

[bjorn3]

Would you mind removing https://github.com/rust-lang/rust/pull/149776/files#diff-b5a09d09fd21f3236a6ae904132f826f38c29be9347281fc97de6792fe081df4R163-R172 in this PR once the subtree sync lands?

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

Some changes occurred in src/tools/compiletest

cc @jieyouxu

[Zalathar]

Removed the now-dead implementation of PrintKind::BackendHasZstd from cg_clif, and added a FIXME comment to llvm_has_zstd in compiletest (diff).