Replace libffi-3.0.11 by libffi-3.1 release

libffi 3.1 was released in May 2014, with the following changes

  - Add AArch64 (ARM64) iOS support.
  - Add Nios II support.
  - Add m88k and DEC VAX support.
  - Add support for stdcall, thiscall, and fastcall on non-Windows
      32-bit x86 targets such as Linux.
  - Various Android, MIPS N32, x86, FreeBSD and UltraSPARC IIi fixes.
  - Make the testsuite more robust: eliminate several spurious
      failures, and respect the $CC and $CXX environment variables.
  - Archive off the manually maintained ChangeLog in favor of git log.

The libffi-3.1.tar.gz tarball was retrieved from

  ftp://sourceware.org/pub/libffi/libffi-3.1.tar.gz

and has the following checksums

md5sum     f5898b29bbfd70502831a212d9249d10
sha1sum    cb373ef2115ec7c57913b84ca72eee14b10ccdc3
sha256sum  97feeeadca5e21870fa4433bc953d1b3af3f698d5df8a428f68b73cd60aef6eb