* add "MIPS_3k_" for the MIPS-I r[23]000-specific register definitions.
* add "MIPS_4k_" for the MIPS-II/III r4000-specific register definitions.
* add #defines that provide the old values for locore and user
code, so the existing code continues to compile.
Regression-tested against the old headers by grepping for #define's,
editing out the defined symbols, and preprocessing with both the previous
machConst.h headers and this version.
Some unused symbols (CPU and FPU must-be-zero constants) are no longer defined.
Pica interrupt masks are now constant expressions instead of constant
values.
TODO:
* factor out the common #defines into src/sys/arch/mips.
* Get rid of the Sprite coding-style names (MACH_xxx).
* Separate out the r3k/r4k differences from the Pica/pmax differences.
* Figure out how to have a run-time choice of r3k vs. r4k support,
instead of a compile-time choice.
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