Building and testing rustc on *BSD fails on -C split-debuginfo

[m-ou-se]

-Csplit-debuginfo is passed on at least NetBSD and FreeBSD when testing rustc, but that results in an error saying that the option is not yet stable.

bootstrap/builder.rs enables unstable options on Linux and Windows, but not on other platforms:

rust/src/bootstrap/builder.rs

Lines 1408 to 1415 in 879fb42

	if target.contains("linux") || target.contains("windows") {
	rustflags.arg("-Zunstable-options");
	}
	match self.config.rust_split_debuginfo {
	SplitDebuginfo::Packed => rustflags.arg("-Csplit-debuginfo=packed"),
	SplitDebuginfo::Unpacked => rustflags.arg("-Csplit-debuginfo=unpacked"),
	SplitDebuginfo::Off => rustflags.arg("-Csplit-debuginfo=off"),
	};

[alex-semenyuk]

@rustbot claim

[luqmana]

This was fixed in #96758