Enable emutls by default for android

[quininer]

This is a split of #117873

This is an attempt to enable emutls by default in android.

The current unresolved problem is that if use emutls provided by compiler-builtins, then each so will have its own emutls symbol during dynamic linking, causing thread_local of each so to be independent of each other.

One solution to this problem is to link against libc++_shared.so provided by android ndk, but this cause in additional runtime dependencies and looks a bit strange.

[rustbot]

r? @cjgillot

(rustbot has picked a reviewer for you, use r? to override)

[bors]

☔ The latest upstream changes (presumably #117873) made this pull request unmergeable. Please resolve the merge conflicts.

[cjgillot]

r? compiler

[TaKO8Ki]

@bors r+ ref: #117873 (comment)

[bors]

📌 Commit 05e4196 has been approved by TaKO8Ki

It is now in the queue for this repository.

[thomcc]

@bors r-

This is the part of the PR that needs careful consideration (and probably FCP!)

[TaKO8Ki]

@thomcc I misunderstood. r? @thomcc

[thomcc]

(Sorry if r-ing a t-compiler PR was out of line, I figured it happened due to a misunderstanding though). It's still a draft too.

Does this version link against libc++? (And if not, do thread locals work in dylibs)

[Dylan-DPC]

@quininer @TaKO8Ki what's the status of this?

[quininer]

There is currently no progress and someone needs to decide how to provide emutls symbols.

I'm already satisfied with the -Ztls-model=emulated flag, so i'm not actively push for this.