Unify target_cpu handling

Over the years several slightly different approaches were used
in the Makefiles to determine the target architecture. This
change updates both the build system and Makefile to handle
this in a consistent fashion.

TARGET_CPU is set to i386, x86_64, powerpc, aarch6 or sparc64
and made available in the Makefiles to be used as appropriate.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9848