remove pthread requirement for cc_target_os:xtensa

PiperOrigin-RevId: 374225894
diff --git a/ruy/build_defs.oss.bzl b/ruy/build_defs.oss.bzl
index e405b41..485d453 100644
--- a/ruy/build_defs.oss.bzl
+++ b/ruy/build_defs.oss.bzl
@@ -11,5 +11,6 @@
     # https://github.com/abseil/abseil-cpp/blob/1112609635037a32435de7aa70a9188dcb591458/absl/base/BUILD.bazel#L155
     return select({
         "@bazel_tools//src/conditions:windows": [],
+        "//tools/cc_target_os:xtensa": [],
         "//conditions:default": ["-pthread"],
     })