pw_boot_cortex_m: Update to support pw_bloat.bloaty_config
pw_boot_cortex_m: Update to support pw_bloat.bloaty_config

Updates the linker script in pw_boot_cortex_m to:
1) Define the symbols necessary for pw_bloat.bloaty_config to
   generate the memoryregions data source.
2) Create sections containing the string unused_space which span
   the end all of the memory regions after the used space (i.e.
   the existing sections).

Change-Id: Id1daefc71e1384f85ff85d366f03715d52bda013
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/83641
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed