Rollup merge of #101873 - WaffleLapkin:x-build-proc-macro-srv, r=jyn514

Rollup merge of #101873 - WaffleLapkin:x-build-proc-macro-srv, r=jyn514

Allow building `rust-analyzer-proc-macro-srv` as a standalone tool

This PR allows building `rust-analyzer-proc-macro-srv` as a standalone tool via `x b proc-macro-srv-cli` (I thought that `x b rust-analyzer-proc-macro-srv` should work, but it doesn't for some reason...). Also this PR adds a copy of `rust-analyzer-proc-macro-srv` binary to `build/{triple}/{stage}/libexec/` when building `rust-analyzer-proc-macro-srv`, so that r-a can pick it up.

This is useful to make r-a (and I assume Intellij IDEA) to expand macros when using a custom, build from source toolchain.

r? ``@jyn514``
[_zulip thread_](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/How.20to.20fix.20.60UnsupportedABI.60.20for.20custom.20toolchains.3F/near/299040175)