Disable fork detection on Trusty.

Trusty doesn't have madvise() or sysconf() and more importantly,
doesn't have fork() (confirmed chatting to ncbray), so the
no-op #if branch in fork_detect.c seems appropriate.

Change-Id: I41b41e79d59919bae6c6ece0e0efd3872105e9b1
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/43204
Commit-Queue: Pete Bentley <prb@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/rand/fork_detect.c

@@ -20,7 +20,7 @@
 
 #include "fork_detect.h"
 
-#if defined(OPENSSL_LINUX)
+#if defined(OPENSSL_LINUX) && !defined(OPENSSL_TRUSTY)
 #include <sys/mman.h>
 #include <unistd.h>
 #include <stdlib.h>