Don't cross-compile Emscripten's LLVM

[alexcrichton]

Eventually the LLVM version of Emscripten and the main LLVM will diverge, and we
can't cross-compile an older version of LLVM using a newer version of LLVM's
llvm-config.

This is extracted from #47828

[rust-highfive]

r? @aturon

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[alexcrichton]

r? @Mark-Simulacrum

[Mark-Simulacrum]

So just to make sure this basically means that we'll be building emscripten LLVM completely (i.e., bootstrapping it) instead of cross compiling from an existing pre-built LLVM?

[alexcrichton]

Nah it's actually a bit better than that! LLVM needs to be told that it's being cross compiled for situations like i686 -> ARM (dunno why). We were informing Emscripten that it was being cross compiled on the i686 builder when we were building from x86_64 -> i686, but this wasn't necessary.

Fuctionally this PR actually changes nothing, we still build LLVM the same number of times (aka Emscripten only once). It just means that when we upgrade LLVM away from Emscripten it'll actually work :)

[Mark-Simulacrum]

Sounds good, thanks!

@bors r+

[bors]

📌 Commit 866d13a has been approved by Mark-Simulacrum

[alexcrichton]

@bors: rollup

[bors]

☔ The latest upstream changes (presumably #47828) made this pull request unmergeable. Please resolve the merge conflicts.

[kennytm]

#47828 (which includes this change) has been merged, closing.