Fix LZ4 endianness autodetection

__BIG_ENDIAN__ is a kernel macro that is only defined when the CPU
is big-endian, but when built in userspace it is always defined.
Let's use different checks depending on whether we're building for
user-space or kernel space.

The "PowerPC" architecture checks were kept just because it's what
Illumos does.

Fixes #1964
Fixed #1965
Signed-off-by: DHE <git@dehacked.net>