Fix LZ4 endianness autodetection

Endianness detection in LZ4 is broken in user-space builds.  This
bug corrupts compressed data and manifests itself in several 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 checks already exist 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 #1963
Fixes #1964
Fixes #1965

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