Bug 1550108 - Pull in secondary lz4 libraries r=glandium

I opted to go with what I perceived as the more expedient route
of leaving lz4 roughly where it is and just adding to that. The
biggest complication was xxhash, which is included elsewhere.
I'm not generally proficient with build-related things though so
my solution may be wrong and not just ugly.

Differential Revision: https://phabricator.services.mozilla.com/D30640

--HG--
rename : mfbt/lz4.c => mfbt/lz4/lz4.c
rename : mfbt/lz4.h => mfbt/lz4/lz4.h
extra : moz-landing-system : lando