m32r port breakage

[Doug Evans]

Ben Elliston writes:
 >   Doug> Why was @PREFIX@_INSN_MAX replaced with "last instruction enum plus one"?
 > 
 > A port I was doing had an instruction called "MAX" that generated the
 > symbol @PREFIX@_INSN_MAX.  Rather than change the name of the sentinel
 > value, I decided to clean this up to avoid any further problems.  I
 > guess I got carried away. :-( Perhaps you could change the sentinel
 > value's name to @PREFIX@_INSN__MAX?

Let's do that.