[roll] Roll fuchsia [netpol] std::sync -> fuchsia_sync

The behaviors of fuchsia_sync's locks are very similar to those from
the standard library. std::sync::Mutex is a port of the libsync
mutex that fuchsia_sync::Mutex links against, and fuchsia_sync::RwLock
is very similar to the std::sync::RwLock's futex-based approach. However
there are some important differences:

1. fuchsia_sync supports lock cycle detection which surfaces many
    deadlocks. See the documentation[0] for more details.
2. fuchsia_sync doesn't support lock poisoning because most of
   Fuchsia's code is compiled with panic=abort.
    a. saves memory with smaller types
    b. avoids noop error handling syntax and branches

[0]: https://fuchsia.dev/fuchsia-src/development/debugging/lock_cycles

Original-Bug: 465178981
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1439908
Original-Revision: d7914a90746abd77df19892c1997361e8a026df6
GitOrigin-RevId: 4d0e42c123a3b93698343a692cb10b8a86c82c51
Change-Id: I4a0bec0736f0bf8740a23ccb7aaa2896a4ce7f96

stem

diff --git a/stem b/stem
index 9e8dc3f..0d90920 100644
--- a/stem
+++ b/stem
@@ -7,7 +7,7 @@
             manifest="manifests/platform"
             root=""
             remote="https://fuchsia.googlesource.com/fuchsia"
-            revision="593071e5697331f1498614196010327944ba3243"/>
+            revision="d7914a90746abd77df19892c1997361e8a026df6"/>
   </imports>
   <projects>
     <project name="cobalt"