rust: restore parallel builds, disable lto only for rustdoc

The original reproducibility fix was problematic for a couple reasons:

- disabling both lto and parallel builds had an unfortunate effect of nearly
doubling rust-native and rust build times (which are slow to begin with).
Disabling lto hurts runtime performance too.

- both of these things were done for the *entire build*, while the only
problematic item is the librustdoc crate.

- lto=off option in config.toml has an effect only on building rustc
(the compiler itself), and doesn't help with rustdoc reproducibility.
Actual fix is the codegen-units setting, which indirectly disables
lto via giving llvm only one unit to work with at a time.

After some digging, here's a more targeted fix for the problem.

Why librustdoc is non-reproducible, but not anything else
remains a mystery, hidden deep in rust-llvm's lto optimization code.

(From OE-Core rev: a1977407a88a2004c3a6d2dba1d5bfe1aa1664b2)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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