Log Message:
Attempt to bring sanity to how ABI and MACHINE_ARCH are set.

Previously there were at least 5 different ways MACHINE_ARCH could be set,
some statically and some at run time, and in many cases these settings
differed, leading to issues at pkg_add time where there was conflict
between the setting encoded into the package and that used by pkg_install.

Instead, move to a single source of truth where the correct value based on
the host and the chosen (or default) ABI is determined in the bootstrap
script.  The value can still be overridden in mk.conf if necessary, e.g.
for cross-compiling.

ABI is now set by default and if unset a default is calculated based on
MACHINE_ARCH.  This fixes some OS, e.g. Linux, where the wrong default was
previously chosen.

As a result of the refactoring there is no need for LOWER_ARCH, with
references to it replaced by MACHINE_ARCH.  SPARC_TARGET_ARCH is also
removed.