Add runtime_spinning_go125_amd64.s to BUILD file

c8e7b67 added the file, but forgot to add it
to the BUILD file, so the package doesn't actually build with Go 1.25

PiperOrigin-RevId: 761198844

Author: prattmic

pkg/sync/BUILD

@@ -33,6 +33,7 @@ go_library(
         "runtime_not_go121_unsafe.go",
         "runtime_other.go",
         "runtime_spinning_amd64.s",
+        "runtime_spinning_go125_amd64.s",
         "runtime_spinning_other.s",
         "runtime_unsafe.go",
         "rwmutex_unsafe.go",