Fix build with clang-cl MSVC 2019 >=16.9.0

After 16.9.0 update of MSVC 2019, this libffi fork stopped building with clang-cl again. This MR fixes it by pulling upstream changes.

• Revert conflicting changes:

  • 6e376edb
  • edaa2f7f

• Merge upstream changes up to https://github.com/libffi/libffi/commit/af6773d6ab4db0577bc6b932ab5a2f98a0a8dca2 These changes are either:

  • Something not used by Meson build (autoconf, test suite)
  • Fix for the issue (only the last commit af6773d6)

Fixes #7

Builds successfully (and seems to work) with cl, clang-cl (MSVC 2019 16.9.0), mingw (cross on Arch Linux)