self-contained linker: conservatively default to -znostart-stop-gc on x64 linux

[lqd]

To help stabilization, this PR disables an LLD optimization on x64 linux with respect to --gc-sections and encapsulation symbols: it will reduce the number of crates needing to opt-out of lld due to this bfd / lld difference. For example, all the people using linkme, which doesn't work with lld or on nightly, need to disable lld.

More information about all this, and the historical differences, can be found in:

• https://maskray.me/blog/2021-01-31-metadata-sections-comdat-and-shf-link-order
• https://lld.llvm.org/ELF/start-stop-gc

This optimization has no visible impact on our benchmarks, so we can use it by default and have a safer/more conservative starting point to remove friction during migration. We can them emit an FCW for the cases where lld detects reliance on encapsulation symbols without -znostart-stop-gc, and then revert back to lld's default after a while. No one compiling on nightly relies on this difference, obviously, so doing an FCW is not necessary until after lld is used on stable.

I've tested that this correctly links on linkme examples. I've also quickly tried to crate an rmake test but the setup with encapsulation symbols is annoying to reproduce: a few link section/name attributes is not enough, we also need to collect symbols between the encapsulation symbols, without referencing them in code, for -znostart-stop-gc to only impact this... It should of course be doable though, maybe @Kobzol will look into it if they have time.

r? @petrochenkov