This consolidates the guts of the three endian.h files (endian.h,
sys/endian.h, and machine/endian.h) into just sys/endian.h, and turns
the others into headers that forward to it.
This also consolidates all the architecture specific machine/endian.h
files into a single generic header.
Finally, this is also the update version of sys/endian.h, which just
forwards to compiler builtins rather than implementing it ourselves.
Change-Id: Ifce64dc684ce3d3231f3f43a94c083bbd8661840
