pw_bloat: Don't use compound assignment on volatile variables
pw_bloat: Don't use compound assignment on volatile variables

Standard doesn't specify behavior of the compound assignment on volatile
variables - it can be either read and write or a single instruction.
Starting with C++20 compilers generate a warning for such cases.

Change-Id: I8679b0162a9b4abbae508e4a7b108b4b5625a390
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98902
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Dennis Kormalev <6222e5e705620133a24041cba003e993480020a9@google.com>
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>

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