sim: mips/ppc/riscv: re-add AC_CANONICAL_SYSTEM [PR sim/29439]

sim: mips/ppc/riscv: re-add AC_CANONICAL_SYSTEM [PR sim/29439]

These configure scripts check $target and change behavior.  They
shouldn't be doing that, but until we can rework the sim to change
behavior based on the input ELF, restore AC_CANONICAL_SYSTEM to
these so that $target is correctly populated.

This was lost in the d3562f83a7b8a1ae6e333cd5561419d3da18fcb4
("sim: unify toolchain probing logic") refactor as the logic was
hoisted up to the common code.  But the fact the vars weren't
passed down to the sub-configure scripts was missed.

Bug: https://sourceware.org/PR29439