riscv64 SCS support.

Bug: https://github.com/google/android-riscv64/issues/55
Test: treehugger
Change-Id: I05d48a07a302305126942d38529ffa280640c7b7
diff --git a/libc/bionic/pthread_exit.cpp b/libc/bionic/pthread_exit.cpp
index bde95ec..f584b27 100644
--- a/libc/bionic/pthread_exit.cpp
+++ b/libc/bionic/pthread_exit.cpp
@@ -117,7 +117,7 @@
     __rt_sigprocmask(SIG_BLOCK, &set, nullptr, sizeof(sigset64_t));
   }
 
-#ifdef __aarch64__
+#if defined(__aarch64__) || defined(__riscv)
   // Free the shadow call stack and guard pages.
   munmap(thread->shadow_call_stack_guard_region, SCS_GUARD_REGION_SIZE);
 #endif