## Why
Windows ARM64 uses 64-bit `long double`, but the LLVM MinGW Bazel
configuration omits the upstream `powl` compatibility source and does
not link the `mingwex` archive that owns it. Cross-linking the release
binary therefore fails with an unresolved `powl` symbol.
## What changed
Patch the LLVM module to compile `math/arm-common/powl.c` into the ARM64
MinGW extension sources and add `-lmingwex` to the Windows toolchain
defaults.
## Validation
- `just bazel-lock-check`
Stack: 3 of 6. Depends on #27322.
Adam Perry @ OpenAI