fix: force simdutf atomic support

Signed-off-by: Matt Leon <mattleon@google.com>

Author: leonm1

bazel/external/simdutf.BUILD

@@ -8,4 +8,5 @@ cc_library(
     name = "simdutf",
     srcs = ["simdutf.cpp"],
     hdrs = ["simdutf.h"],
+    defines = ["SIMDUTF_ATOMIC_REF=1"],
 )