Enable thread support for Zephyr RTOS

Zephyr RTOS supports multithreading with preemptive scheduling, so we
must enable support for threads in BoringSSL

BUG=b:321092852

Change-Id: I6fadce72e71ac043513fda0a5639261ba396b7f5
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/67628
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/target.h

diff --git a/include/openssl/target.h b/include/openssl/target.h
index 8b05661..2760f52 100644
--- a/include/openssl/target.h
+++ b/include/openssl/target.h
@@ -152,12 +152,15 @@
 // Defining this on any other platform is not supported. Other embedded
 // platforms must introduce their own defines.
 //
-// https://chromium.googlesource.com/chromiumos/platform/ec/+/HEAD/docs/zephyr/README.md
+// Zephyr supports multithreading with cooperative and preemptive scheduling.
+// It also implements POSIX Threads (pthread) API, so it's not necessary to
+// implement BoringSSL internal threading API using some custom API.
+//
+// https://www.zephyrproject.org/
 #if defined(__ZEPHYR__)
 #define OPENSSL_NO_FILESYSTEM
 #define OPENSSL_NO_POSIX_IO
 #define OPENSSL_NO_SOCK
-#define OPENSSL_NO_THREADS_CORRUPT_MEMORY_AND_LEAK_SECRETS_IF_THREADED
 #endif
 
 #if defined(__ANDROID_API__)