Add support for rust.lto=off to bootstrap for faster dev builds

[the8472]

Currently bootstrap supports LTO flavors thin-local (default), thin and fat. Adding the option to completely disable it can speed up develop cycles, especially with incremental compilation. See this Zulip thread for benchmarks.

rust/src/bootstrap/config.rs

Lines 334 to 341 in 8e68090

	/// LTO mode used for compiling rustc itself.
	#[derive(Default, Clone)]
	pub enum RustcLto {
	#[default]
	ThinLocal,
	Thin,
	Fat,
	}

When LTO is disabled the standard library still has to be built with -Cembed-bitcode=on so that it can be used by other crates which enable LTO.

Disabling should become the default for the library and compiler profile templates.

[albertlarsan68]

@rustbot claim to not loose it to time, but feel free to claim it if you want, I might not start work on this until February.

[rustbot]

Error: Parsing assign command in comment failed: ...' claim' | error: expected end of command at >| ' to not lo'...

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[albertlarsan68]

@rustbot claim
Shouldn't this bot be able to detect end of command and silently ignore the rest of the line ?