Ignore proc-macros when assembling rustc libdir

[tmiasko]

Fixes #80294.

[rust-highfive]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

I just re-read the explanation of dual proc macros - https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/-Z.20dual-proc-macros.

cc @petrochenkov

I think this should be fine, presuming my model that $sysroot/lib/ exists only for runtime linkage of rustc (and tool) binaries is accurate. The proc macro libraries dropped by this PR are currently still needed in the $sysroot/lib/rustlib/$host/lib and $sysroot/lib/rustlib/$target/lib when cross-compiling plugins/drivers from $host to $target, but $sysroot/lib should not be in the compiler's search path at that time I believe (but am somewhat unsure). AFAICT, this PR should not change the contents of the rustlib directory.

[petrochenkov]

I more or less remember why -Zdual-proc-macros are necessary, but have very little idea of how they maps on files and directories in Rust distribution, and which of those files are affected by this patch.

If both proc-macros-for-target and proc-macros-for-host were previously shipped together with other-artifacts-for-target, and now only proc-macros-for-target are shipped, then it should be ok.

[petrochenkov]

In any case, this can be reverted later if something breaks.

[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit 3271681 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 3271681 with merge 76188b6...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 76188b6 to master...