[Compilation] Allow ThinLTO and Fix CI flakiness

[Skylion007]

Motivation and Context

• I just tested the latest LLVM (13.0) and it fixes the segfault that prevented us from using using ThinLTO compilation.
• Also tries to fix a flaky portion of the CI
• I can remove the cludge for EMSCRIPTEM (which is already at a supported LLVM version number), once my toolchains PR is merged in (Properly set EMAR for Interprocedural Optimization mosra/toolchains#14) Ping @mosra

How Has This Been Tested

• Tested with LLVM Clang 13.0

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• All new and existing tests passed.

[jturner65]

LGTM