Rollup merge of #143255 - Kobzol:disable-lld-by-default, r=jieyouxu

Do not enable LLD by default in the dist profile

History of us building & shipping LLD for `dist` builds:
1) We used to unconditionally build & ship LLD in bootstrap
2) This was causing problems for people doing custom `dist` builds (https://rust-lang.zulipchat.com/#narrow/stream/326414-t-infra.2Fbootstrap/topic/MSVC.20Runtime.20mismatch.20when.20building.20LLD)
3) https://github.com/rust-lang/rust/pull/126701 made shipping of LLD optional, but to preserve previous behavior, it forcefully enabled `rust.lld = true` in the `dist` profile by default, and overwrote the default to `false` on our CI for external LLVM builds.
    - This also didn't match the documentation of `rust.lld` in `bootstrap.example.toml`, which I previously missed.
4) However, since the external LLVM opt-out was only implemented for our CI, and not for all `dist` users, this started causing issues for people `dist`ing with external LLVM (https://github.com/rust-lang/rust/issues/143076). The problem is that the default shouldn't be "true", but "LLD is enabled when LLVM isn't external", but this is not possible to do only in TOML.

So this PR reverses the behavior. LLD is not enabled by default in `dist` anymore. We switch our CI to *opt into* disting LLD, unless an external LLVM is used. External `dist` users can still opt into enabling LLD, but if they do so while also using external LLVM, they will now get a [hard error](https://github.com/rust-lang/rust/pull/143175).

r? `@jieyouxu`

try-job: `x86_64-mingw*`
try-job: dist-x86_64-linux

0 files changed, 0 insertions, 0 deletions