mmix: support -fstack-usage

mmix: support -fstack-usage

MMIX has two stacks; the regular one using register $254 as a
convention and the register-stack, pushed and popped by call
instructions (usually).  The decision to only report the stack usage
of the regular stack (and not of the register stack) may be updated,
perhaps the sum is better.  This initial decision is helped a little
bit by the order of passes: the size of the register-stack is
calculated only later (in the machine-dependent reorg pass), long
after finalization of the stack-usage info (in the prologue/epilogue
pass).  No regressions for mmix-knuth-mmixware (but a whole lot more
PASSes), committed.

gcc:
	* config/mmix/mmix.c (mmix_expand_prologue): Calculate the total
	allocated size and set current_function_static_stack_size, if
	flag_stack_usage_info.