runners: bench: Moved BENCH_PERBYTE to runtime (DISK_SIM=1)

If only for consistency with DISK_GEOMETRY.

The main reason to keep BENCH_PERBYTE around is to help debug/sanity
check the more complex bus+buffer sim. For that purpose it makes sense
to be able to easily switch modes.

The only downside is if it's more difficult to introduce -DDISK_SIM=1 at
runtime vs compile-time, but eh. Consistency wins.

Makefile

@@ -124,9 +124,6 @@ BENCH_CFLAGS += -Wl,--wrap=malloc
 BENCH_CFLAGS += -Wl,--wrap=free
 BENCH_CFLAGS += -Wl,--wrap=realloc
 endif
-ifdef BENCH_PERBYTE
-BENCH_CFLAGS += -DBENCH_PERBYTE
-endif
 
 ifdef VERBOSE
 CODEFLAGS    += -v
@@ -203,6 +200,9 @@ endif
 ifdef BENCH_NAND
 BENCHFLAGS += -DDISK_GEOMETRY=1
 endif
+ifdef BENCH_PERBYTE
+BENCHFLAGS += -DDISK_SIM=1
+endif
 
 # this is a bit of a hack, but we want to make sure the BUILDDIR
 # directory structure is correct before we run any commands