Allow linking a prebuilt optimized compiler-rt builtins library

[pmur]

Extend the .optimized-compiler-builtins bootstrap option to accept a path to a prebuilt compiler-rt builtins library, and update compiler-builtins to enable optimized builtins without building compiler-rt builtins.

[rustbot]

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[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

[Kobzol]

I don't really know much about the optimized compiler-rt builtins. Maybe @tgross35 might know more?

[Mark-Simulacrum]

Can you say more about how much time this saves? Historically it's been my impression locally that building is basically free, this flag is more about whether it's possible to build (e.g., needs to be off if you're ad-hoc cross compiling). Though maybe I'm remembering wrong?

[pmur]

Can you say more about how much time this saves? Historically it's been my impression locally that building is basically free, this flag is more about whether it's possible to build (e.g., needs to be off if you're ad-hoc cross compiling). Though maybe I'm remembering wrong?

The goal is to enable optimized compiler-builtins in fedora packaging. There, rust is built using the distro provided llvm and resulting libraries. Some runtime features are absent if optimized compiler-builtins are not used. Notably, outlined aarch64 LSE atomic operations are not available without this support.

[cuviper]

Right, it's less about time and more about wanting to use our existing system LLVM packages.