Auto merge of #57130 - VardhanThigle:Vardhan/x86_64-fortanix-unknown-sgx-tier2_support, r=alexcrichton

Upgrade x86_64-fortanix-unknown-sgx platform support to tier 2

## Overview
1. This PR upgrades x86_64-fortanix-unknown-sgx platform support to tier 2 (std only) by setting up build automation for this target.
1. For supporting unwinding, this target needs to link to a port of LLVM's libunwind (more details could be found in #56979), which will be distributed along with the Rust binaries (similar to the extra musl objects)

### Building and copying libunwind:
We have added a new build script  (`build-x86_64-fortanix-unknown-sgx-toolchain.sh`) that will run while the container is built. This will build `libunwind.a` from git source.
While the container is built, the persistent volumes where obj/ gets created aren't yet mapped. As a workaround, we copy the built `libunwind.a` to  `obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-fortanix-unknown-sgx/lib/` after x.py runs.
 If any reviewer knows of a better solution, please do tell.

r? @Mark-Simulacrum

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