libcompiler_builtins: Don't build emutls.c

[jdub]

Rather than improving the check, let's ditch emutls.c entirely.

[rust-highfive]

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[jdub]

I double-checked that rustc +nightly --print cfg for Windows (both MSVC and GNU) doesn't contain target_thread_local, but I'm not sure about Redox.

It correctly builds without emutls.c on my bare metal target, and with emutls.c on Linux with nightly.

[nikomatsakis]

r? @alexcrichton

I have no idea what this is about. =)

[alexcrichton]

@jdub in theory we never actually even use this file and we should actually stop compiling it. Did you find this necessary to fix something though?

[jdub]

Yeah, it fails to build on my non-Redox, non-Windows target because it's a bare metal target without POSIX/pthreads. 😄 If emutls.c is truly unnecessary (about which I can only have blind trust!), should I update this PR to ditch it?

[alexcrichton]

Yeah let's just update this to avoid compiling it entirely

[sanxiyn]

You may want to edit the first comment which is included as a commit message by bors, since it is now incorrect and probably will confuse future readers of git log.

[jdub]

@sanxiyn Thanks -- done.

[sanxiyn]

@bors r+ rollup

[bors]

📌 Commit 9f92d4f has been approved by sanxiyn