Use ELF-TLS on SGX

[joboet]

fortanix/rust-sgx#419 adds support for thread-local storage to the ELF loader. This allows std to get rid of the key allocation interface so it only needs to manage initializing the TLS section and running destructors on thread exit.

r? @jethrogb
(I totally understand if you don't want this or don't have the time to review. I was just having a bit of fun researching how TLS works 😄.)

@rustbot label +T-libs +O-SGX

[rustbot]

Failed to set assignee to jethrogb: invalid assignee

Note: Only org members, users with write permissions, or people who have commented on the PR may be assigned.

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[apiraino]

r? libs

[joboet]

@rustbot label +S-blocked
as it depends on fortanix/rust-sgx#419

[bors]

☔ The latest upstream changes (presumably #106511) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #117716) made this pull request unmergeable. Please resolve the merge conflicts.