Use --eh-frame-hdr on linux-musl target & ld.lld linker flavour

[tmiasko]

The --eh-frame-hdr is necessary for unwinding but not enabled by default
when using lld.ld linker flavour. Enable it to match existing behaviour
for gcc linker flavour.

Closes #73451.

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[petrochenkov]

I'm not sure this will work as expected because non-baremetal targets don't really support linking with a "naked" linker (as opposed to the gcc wrapper) out of the box.

What the author of #73451 wanted to do is actually -C link-arg -fuse-ld=lld.
Then lld will be used through gcc (or clang) and --eh-frame-hdr will be passed.

[tmiasko]

I know that this approach doesn't work with linux-gnu targets for reasons you
described (it needs additionally --eh-frame-hdr, -dynamic-linker, paths to
system libraries, crt, ...), but it does work with musl.

[petrochenkov]

It works with musl, but only in self-contained mode (see #68887 (comment) for details), which should ideally be enabled only when a native musl toolchain is nowhere in sight.

I wouldn't want to make it work today just break it tomorrow by changing the self-contained mode heuristic.

[petrochenkov]

Speaking of --eh-frame-hdr specifically, it's one of those options that we should probably pass unconditionally in link.rs rather than in target specs (to linkers that support it).

[tmiasko]

It is already possible to use -Clinker-flavor=ld.lld -Clink-arg=--eh-frame-hdr,
so any hypothetical heuristics with regards to self contained mode would be to
some degree a regression regardless.

So, what do you propose?

1. Close this now and potentially revisit once self contained mode option / heuristic is implemented.
2. Explore using --eh-frame-hdr more generally through link.rs.
3. Land it as is.
4. ...

[alecmocatta]

What the author of #73451 wanted to do is actually -C link-arg -fuse-ld=lld.
Then lld will be used through gcc (or clang) and --eh-frame-hdr will be passed.

Just to clarify, I'd like to be able to cross-compile from Windows. -C linker=rust-lld is the way I've been recommended to do this. I'm not sure what it would take to make -C link-arg -fuse-ld=lld work but it doesn't work out of the box.

[petrochenkov]

Let's try

2. Explore using --eh-frame-hdr more generally through link.rs.

(Closing in favor of #73564.)