Support TLS

Nobody has claimed this yet.

I've migrated a D codebase from the riscv/riscv-llvm -based compiler to the lowRISC/riscv-llvm one. After the latest lowRISC/riscv-llvm changes I was able to remove essentially all workarounds for invalid code (although the latest changes (because of the new branch analysis?) seemed to cause a 4X performance penalty, despite allowing more modules to compile without -O0).

The only major issue remaining for great D support in this LLVM backend is the lack of thread-local storage. Since D globals default to TLS, that lack is particularly felt. I'm opening this issue to communicate the relative importance of adding TLS. Please address that when possible.

Start with the riscv-llvm backend and investigate how thread-local storage is represented and supported in the LLVM-based compiler. Use the issue's D globals and comparison with riscv/riscv-llvm as context, then determine the required implementation scope and validation needed for TLS support.

Written by the indexing model from the issue text.