Clean master's content to merge in bits ARC needs.

external/libffi is not used by anyone in master except ARC++ team, and
ARC++ team needs to use a different version of libffi. Therefore it's OK
to clean up old content, and merge in the version of libffi ARC++ uses.

Note external/libffi is not part of master's manifest.

Test: "ls -a" only shows ". .. .git".
Change-Id: I769e810d1ed284995f259043e8d13f74fd50ab7d