Adds initial ThreadX support through a set of backends for pw_sync
through pw_sync_threadx and pw_chrono through pw_chrono_threadx.
Change-Id: I3e310323cfce3d4bf07557273c2a86f91f6479d7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/24900
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Ewout van Bekkum <ewout@google.com>
