Upstream LLVM libunwind patch for SGX

[nikic]

Our LLVM fork currently carries a single patch, which adds support for SGX to libunwind: rust-lang/llvm-project@29e82b2

This patch should be upstreamed to LLVM (in some form that is acceptable for upstream).

cc @jethrogb @AdrianCX

[joboet]

Alternatively, SGX could use the unwinding crate like Xous currently does.

[raoulstrackx]

I'll look into this