pw_bloat: By default, don't print size reports

Size reports currently print when they're generated. This creates a fair
amount of noise which will only increase over time. Add the
pw_bloat_SHOW_SIZE_REPORTS build arg that controls whether to print
size reports in the build.

Change-Id: I82a2578e5412cf20c31f70d5eba058f84697f0a3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/44820
Reviewed-by: Alexei Frolov <frolv@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>