third_party/freertos: Don't use -Wno-error
third_party/freertos: Don't use -Wno-error

Changes freertos GN build files to use -Wno-* flags rather than
-Wno-error=* flags so warnings are completely silenced rather than
forced to be warnings.

Change-Id: Iea9aa58681402171173a14b74e681f035b34f229
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/73560
Pigweed-Auto-Submit: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

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