Fix Windows native voice dependency builds

[copyberry]

Fix Windows native voice dependency builds

Why

Windows builds combine Cygwin build tools with native MSVC outputs. On ARM64,
Cygwin can run under x64 emulation, so inferred host details and untranslated
paths can select the wrong target or leak POSIX paths into native metadata.

What changed

• Convert libffi source, prefix, and shell paths with cygpath, preserve
  USERPROFILE, and pass explicit build and host targets.
• Configure libffi's MSVC and libtool environment to produce and install its
  shared library and import library, including across recursive make calls.
• Reject libffi pkg-config output containing /cygdrive/ paths and enable
  Opus NEON support for aarch64-pc-windows-msvc.
• Document the additional Windows build prerequisites.

Testing

• Extend test_build_native.py with Windows path conversion, x64 and ARM64
  target configuration, recursive make flag propagation, pkg-config, and Opus
  coverage.

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