rustdoc ignores cargo and linker settings when compiling doc tests

[alice-i-cecile]

When building doc tests, cargo settings like --jobs are ignored, and the linker specified in config.toml is also ignored.

However, by setting rustdocflags we can control the linker correctly. This should use the project-level linker by default.

Context

These limitations make problems with slow / high memory doc test construction (see #75341) much harder to mitigate.
The approach proposed in #51228 may offer a way to fix this.

Initially discovered by Giooschi on the Bevy Discord while investigating bevyengine/bevy#12207.

Meta

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-pc-windows-msvc
release: 1.78.0
LLVM version: 18.1.2

[ehuss]

I believe linker support is gated behind the doctest-xcompile feature.

I think jobs support is already tracked as a request in rust-lang/cargo#10702 and #53143.