pw_bluetooth_sapphire: Configure interrupt spin lock backend for Fuchsia

Change-Id: If2bf242c729a8381fb2e492d6c42c1d7e2f2c328
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/247773
Docs-Not-Needed: Ben Lawson <benlawson@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Reviewed-by: Taylor Cramer <cramertj@google.com>
Commit-Queue: Ben Lawson <benlawson@google.com>

pw_bluetooth_sapphire/fuchsia/backends.bazelrc

@@ -29,6 +29,7 @@ common:fuchsia --@pigweed//pw_log:backend_impl=@pigweed//pw_build:empty_cc_libra
 common:fuchsia --@pigweed//pw_unit_test:backend=@pigweed//pw_unit_test:googletest
 common:fuchsia --@pigweed//pw_async:task_backend=@pigweed//pw_async_fuchsia:task
 common:fuchsia --@pigweed//pw_async:fake_dispatcher_backend=@pigweed//pw_async_fuchsia:fake_dispatcher
+common:fuchsia --@pigweed//pw_sync:interrupt_spin_lock_backend=@pigweed//pw_sync_stl:interrupt_spin_lock
 
 # TODO: https://pwbug.dev/374340793 - This is a temporary workaround for now and
 # this command line option shouldn't be applied more widely.

pw_sync/public/pw_sync/yield_core.h

@@ -22,6 +22,10 @@
 #elif defined(__aarch64__) || defined(__arm__)
 #define PW_SYNC_YIELD_CORE_FOR_SMT() asm volatile("yield" ::: "memory")
 
+#elif defined(__Fuchsia__)
+#include <zircon/syscalls.h>
+#define PW_SYNC_YIELD_CORE_FOR_SMT() zx_thread_legacy_yield(0)
+
 #else
 #error "No processor yield implementation for this architecture."