Fix LZ4 endianness autodetection

Endianness detection in LZ4 is broken in user-space builds.  This
bug corrupts compressed data and manifests itself in at least two
obscure ztest failures.  When LZ4 was originally ported to Illumos
ZFS, the proper checks for Linux were stripped out. The Linux port
then inherited the remaining detection code that works on Illumos
but not on Linux.

The current LZ4 endianness check misuses the condition
defined(__BIG_ENDIAN) to indicate a big-endian system.  On Linux
__BIG_ENDIAN is defined uncondtionally in the user-space header
/usr/include/endian.h, regardless of the endianness of the system.
The kernel does not use this header, so only user-space builds are
affected.

While we could fix this by restoring the upstream LZ4 endianness
detection code, reliable detection code already exists in
libspl/include/sys/isa_defs.h. This change uses the libspl results
to replace the word-size and endianness checks in LZ4, simplifying
the code and reducing duplication.

Fixes #1964
Fixes #1965

Signed-off-by: Ned Bass <bass6@llnl.gov>