Log Message:
libffcall: update to 2.4.

New in 2.4:

* Added support for the following platforms:
  (Previously, a build on these platforms failed.)
  - arm64: macOS 11.
  - arm: FreeBSD 12.
  - sparc: Solaris 11 32-bit.

New in 2.3:

* Added support for the following platforms:
  (Previously, a build on these platforms failed.)
  - x86_64: FreeBSD 13, OpenBSD 6.5 and newer.
  - powerpc64: AIX 7 with xlclang compiler.

* Fixed a couple of multithread-safety bugs in the trampoline and callback
  packages.