runners: A number of stack/heap measurement tweaks

- Renamed BENCH_STACK/HEAP -> BENCH_STACK/HEAP_WATERMARK

- Renamed BENCH_YES_STACK/HEAP -> BENCH_STACK/HEAP

- Tweaked stack/heap watermarks to hopefully be easier to access when
  debugging. Now also exposed as global variables
  (bench_stack/heap_watermark).

  I considered changing BENCH_STACK/HEAP_WATERMARK to be the variable
  itself, to be consistent with TEST_PLS, but decided against it:

  1. BENCH_STACK_CURRENT() is a bit magic in that it relies on
     __attribute__((noinline)) to force a new stack frame. This wouldn't
     really be possible with a variable.

  2. TEST_PLS is at least constant from the _current run_'s perspective.
     This isn't true for the stack/heap watermarks.

- Reworked internals a bit to hopefully be simpler

Makefile

@@ -104,10 +104,10 @@ CFLAGS += $(foreach d,$(filter LFS3_%,$(.VARIABLES)),-D$d=$($d))
 TEST_CFLAGS += -Wno-unused-function
 TEST_CFLAGS += -Wno-format-overflow
 ifdef STACK
-TEST_CFLAGS += -DTEST_YES_STACK
+TEST_CFLAGS += -DTEST_STACK
 endif
 ifdef HEAP
-TEST_CFLAGS += -DTEST_YES_HEAP
+TEST_CFLAGS += -DTEST_HEAP
 TEST_CFLAGS += -Wl,--wrap=malloc
 TEST_CFLAGS += -Wl,--wrap=free
 TEST_CFLAGS += -Wl,--wrap=realloc
@@ -116,10 +116,10 @@ endif
 BENCH_CFLAGS += -Wno-unused-function
 BENCH_CFLAGS += -Wno-format-overflow
 ifndef NO_STACK
-BENCH_CFLAGS += -DBENCH_YES_STACK
+BENCH_CFLAGS += -DBENCH_STACK
 endif
 ifndef NO_HEAP
-BENCH_CFLAGS += -DBENCH_YES_HEAP
+BENCH_CFLAGS += -DBENCH_HEAP
 BENCH_CFLAGS += -Wl,--wrap=malloc
 BENCH_CFLAGS += -Wl,--wrap=free
 BENCH_CFLAGS += -Wl,--wrap=realloc