Cherry-picked upstream out-of-order emubd testing

More information upstream (f2a6f45, fc2aa33, 7873d81), but this adds
LFS_EMUBD_POWERLOS_OOO for testing out-of-order block devices that
require sync to be called for things to serialize. It's a simple
implementation, just reverts the first write since last sync on
powerloss, but gets the job done.

Cherry-picking these changes required reverting emubd's scratch buffer,
but carrying around an extra ~block_size of memory isn't a big deal
here.

runners/bench_runner.h

@@ -157,7 +157,8 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
 #define BENCH_BDCFG \
     .erase_value        = ERASE_VALUE,          \
     .erase_cycles       = ERASE_CYCLES,         \
-    .badblock_behavior  = BADBLOCK_BEHAVIOR,
+    .badblock_behavior  = BADBLOCK_BEHAVIOR,    \
+    .powerloss_behavior = POWERLOSS_BEHAVIOR,
 
 
 #endif