Enforce the compiler-builtins partitioning scheme

[saethlin]

In progress still, I want to at least add a test that checks that we get the partitioning right when using -Zbuild-std. And add/improve comments.

compiler-builtins needs every intrinsic in its own CGU. Currently, the compiler-builtins crate puts every intrinsic in its own inline module then library/Cargo.toml uses a profile override so that when we build the sysroot, compiler-builtins is built with more codegen-units than we have intrinsics, and partitioning never merges two intrinsics together. This approach does not work with -Zbuild-std because the profile override gets ignored. And it's kludgey anyway, our own standard library should not be fighting with our own compiler in an attempt to override its behavior. We should change the behavior to do the right thing in the first place.

So that's what this PR does. There's some light refactoring of the CGU partitioning code, then in 3 places I've added a check for is_compiler_builtins:

• There's a special case now in cross_crate_inlinable; every function in compiler-builtins that is not #[no_mangle] is made cross-crate-inlinable, which ensures we do not run into problems inlining helpers into intrinsics such as compiler-builtins: Int trait functions are not inlined on wasm #73135
• When building compiler-builtins, the name of the CGU that a MonoItem is given is just the MonoItem's symbol name. This puts every GloballyShared item in its own CGU.
• Then when building compiler-builtins, we skip CGU merging.

That should ensure that we have one object file per intrinsic, and if optimizations are enabled, there should be no extra extra CGUs full of helper functions (which is what currently happens in the precompiled standard library we distribute).

r? ghost