Fix requires_lto targets needing lto set in cargo

[Flakebi]

Targets that set requires_lto = true were not actually using lto when compiling with cargo by default. They needed an extra lto = true in Cargo.toml to work.

Fix this by letting lto take precedence over the embed_bitcode flag when lto is required by a target.

If both these flags would be supplied by the user, an error is generated. However, this did not happen when lto was requested by the target instead of the user.

Fixes #148514
Tracking issue: #135024

[rustbot]

r? @jieyouxu

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[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[jieyouxu]

Hm, not entirely sure of the implications.
@rustbot reroll

[Flakebi]

The CI/tidy complain seems like a false-positive, the test uses --target, just not through //@ compile-flags:. Maybe that lint shouldn’t run on run-make-cargo tests?
(I’m happy to make the change, just want to get confirmation that it’s ok to do that.)

[jieyouxu]

The CI/tidy complain seems like a false-positive, the test uses --target, just not through //@ compile-flags:. Maybe that lint shouldn’t run on run-make-cargo tests? (I’m happy to make the change, just want to get confirmation that it’s ok to do that.)

Yes, I probably forgot to extend the tidy exception to run-make-cargo when I split the previous run-make test suite.