Adds (most) support for ARC platform to port-independent mips code.
Some changes (e.g., clean up of overlapping CPU/FPU ids) inspired by
comparison to the OpenBSD 2.1 codebase of Soda's ARC port.
Open issues:
* Still no support for r4600 or mipsIV CPUs with two-way L1 cache.
Code derived from Per Fogelstrom's OpenBSD source doesn't work
on mips3 pmaxes with L2 cache.
* Still some port-specific #ifdefs, for interrupt enable and
pmax L2 cache-size. Needs more thought, but overlaps with
work-in-progress by Tohru and Tsubai on spl()s and related stuff.
* cpuregs.h:
Delete unused VMMACH_ names (e.g., duplicates of PTE bits in pte.h).
Change remaining VMMACH_xxx names to MIPS1_xxx or MIPS3_xx.
Fold remaining compile-time definitions into a single #ifdef MIPS3.
* Use MIPS1_ names in locore_r2000.S, mips3_ names in locore_r4000.S
* Garbage-collect MachHitFlushDCache()
* psl.h:
use MIPS1_, MIPS3_ symbolic names for Cause register bits.
change _R3K to MIPS1_, _R4K to MIPS3. Conditionalize for mips1 only,
mips3 only, or when both are defined, use runtime CPUISMIPS3 test.
Change pmax/include/psl.h to just do #include <mips/psl.h>.
pmax/include/psl.h would go away completely if it wasn't stil required
by compat/common/kern_exit_43.c.
jonathan