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>

meta/recipes-devtools/rust/rust-source.inc

@@ -4,10 +4,10 @@ SRC_URI += "https://static.rust-lang.org/dist/rustc-${RUST_VERSION}-src.tar.xz;n
             file://zlib-off64_t.patch;patchdir=${RUSTSRC} \
             file://rv32-rustix-libc-backend.patch;patchdir=${RUSTSRC} \
             file://rv32-cargo-rustix-0.38.28-fix.patch;patchdir=${RUSTSRC} \
-            file://rustdoc-repro-issue-fix-cargo-config-for-codegenunits.patch;patchdir=${RUSTSRC} \
             file://rust-oe-selftest.patch;patchdir=${RUSTSRC} \
             file://repro-issue-fix-with-cc-crate-hashmap.patch;patchdir=${RUSTSRC} \
             file://oeqa-selftest-Increase-timeout-in-process-sigpipe-ru.patch;patchdir=${RUSTSRC} \
+            file://0001-src-core-build_steps-tool.rs-switch-off-lto-for-rust.patch;patchdir=${RUSTSRC} \
 "
 SRC_URI[rust.sha256sum] = "36217ef7e32f40a180e3d79bd666b4dfdaed49dd381023a5fb765fd12d0092ce"