health: build the instrumentation only when asked for

The counters, the phase hooks in idle() and the "health" command are now
behind HEALTH, set from the command line with HEALTH=1. Without it the
hooks are empty macros and the command does not exist, so a default build
carries none of it: on SWTGW218AS that is 1065 bytes of bank 2, 101 of the
common segment and 28 of xdata, plus seven banked calls per main-loop pass.

It is a developer's tool. Reading the dump takes knowing the main loop, and
a user who needs it for a report can build with the flag on, which is how
the kernel treats its lock and latency instrumentation as well.

Makefile

@@ -19,6 +19,10 @@ ifeq ($(MACHINE),)
 else
 	CC_FLAGS += -DMACHINE_$(MACHINE)
 endif
+# Health instrumentation and the "health" console command: HEALTH=1 gmake ...
+ifneq ($(HEALTH),)
+	CC_FLAGS += -DHEALTH
+endif
 
 ifeq ($(CI),1)
 	CC_FLAGS += --Werror