Fix LTO regressions in nightly toolchain

Sergej Kostjucenko requested to merge aj-xbuild into master

Created by: ascjones

Fix for https://github.com/paritytech/ink/issues/391.

Recent nightly has broken since https://github.com/rust-lang/rust/pull/71925 (see https://github.com/rust-osdev/cargo-xbuild/issues/69#issuecomment-627231724)

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