Make libffi install into a predictable directory (#9620)

On some systems (depending on gcc multilib configuration) libffi
would install into libffi/build/inst/lib64 even though we configure
it with --libdir=libffi/build/inst/lib. There appears to be no way
to get libffi to install to a predictable directory "out of the box",
so we apply a small patch to Makefile.in. This is the same fix used
in Gentoo's ebuild (https://bugs.gentoo.org/show_bug.cgi?id=462814).