[WIP] Use weak linkage instead of compiler generated shims

[bjorn3]

This is still keeping the allocator shim for the oom handler and the __rust_no_alloc_shim_is_unstable symbol for now.

TODO: Update comments everywhere, test on macOS and Windows and write a better PR description why we want this. Also prevent codegen of weak symbols when there is a non-weak definition of the same symbol in the same codegen unit.

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

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failures:

---- library/core/src/alloc/global.rs - alloc::global::GlobalAlloc (line 22) stdout ----
error: the `#[global_allocator]` in this crate conflicts with global allocator in: std
error: aborting due to 1 previous error

Couldn't compile the test.