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
