Support Trusty, an embedded platform.

Trusty doesn't have setjmp.h and nor does it have threads.

Change-Id: I005f7a009a13e6632513be9fab2bbe62294519a4
Reviewed-on: https://boringssl-review.googlesource.com/4660
Reviewed-by: Adam Langley <agl@google.com>

crypto/CMakeLists.txt

diff --git a/crypto/CMakeLists.txt b/crypto/CMakeLists.txt
index 06cbd44..5927a05 100644
--- a/crypto/CMakeLists.txt
+++ b/crypto/CMakeLists.txt
@@ -144,6 +144,7 @@
   crypto.c
   mem.c
   thread.c
+  thread_none.c
   thread_pthread.c
   thread_win.c
   ex_data.c