Experiment: Try making all compiler-builtins intrinsics weak symbols

[Amanieu]

This is an experiment to try making all compiler-builtins symbols weak by default. This should solve issues like rust-lang/compiler-builtins#353 and rust-lang/compiler-builtins#420 where our symbols conflict with those from libgcc/libclang_rt.

r? @ghost

[Amanieu]

The issue on Windows seems to be that weak symbols can be defined, but not dllexport'ed. This causes problems when building the standard library as a DLL since compiler_builtins is placed in it, and the rustc_driver DLL imports the compiler_builtins functions from it.

[mati865]

Forwarding from rust-lang/compiler-builtins#546 (comment)

Weak symbols on Windows with ld.bfd are a can of worms.
Generally you can expect them to rather work as long as you are combining static archives with weak symbols into one final binary/library. Trying to do the same with shared libraries might work for simplest cases but generally think of it as broken.
You may have more luck with __declspec(selectany) but it will use more COMDATs which Windows limits to 2^16.

LLD is has working weak symbols (in both static and shared cases) but we cannot use it because of another Binutils bug import libraries created by LLD break symbol table with Binutils <2.40.

[bors]

☔ The latest upstream changes (presumably #116196) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC]

Closing this as it was an experiment