Fix powerpc build

Unlike other architectures which sanitize the LDFLAGS from the
environment in arch/<arch>/Makefile. The powerpc Makefile
allows LDFLAGS to be passed through resulting in the following
build failure.

  /usr/bin/ld: unrecognized option '-Wl,-z,relro'

LDFLAGS is set in /usr/lib/rpm/redhat/macros by default. Clear
the environment variable when building kmods for powerpc.

Additionally, now that ppc64le exists it's not longer safe to
assume a powerpc system is big endian. Rely on the endianness
provided by the compiler.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5856