Switch to <stdbool.h> in the VM.

Switch to <stdbool.h> in the VM.

We were using an enum that made the compiler unhappy on MacOS X.  This
switches us to using <stdbool.h> when it's available.

The size of a "bool" is either sizeof(_Bool) or sizeof(enum bool), and
the assembly sources dislike ambiguity, so this also changes
gDvm.debuggerActive to always be a single byte.  The ARM and x86 code
was already assuming that -- held over from when enums were
variable-width, and never fixed because we get the right answer on
little-endian platforms -- so it doesn't look like we need to change
anything in mterp.