third_party/fuchsia: Sync bit.h from stdcompat
third_party/fuchsia: Sync bit.h from stdcompat

A pending change to fit::function will use bit.h, and <bit> polyfills
are already needed in Pigweed to replace __builtin_clzll.

Change-Id: Iab955eb7060634f19462969a9004c50d6114c39b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/113910
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@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