Add visibility rules.

This change marks public symbols as dynamically exported. This means
that it becomes viable to build a shared library of libcrypto and libssl
with -fvisibility=hidden.

On Windows, one not only needs to mark functions for export in a
component, but also for import when using them from a different
component. Because of this we have to build with
|BORINGSSL_IMPLEMENTATION| defined when building the code. Other
components, when including our headers, won't have that defined and then
the |OPENSSL_EXPORT| tag becomes an import tag instead. See the #defines
in base.h

In the asm code, symbols are now hidden by default and those that need
to be exported are wrapped by a C function.

In order to support Chromium, a couple of libssl functions were moved to
ssl.h from ssl_locl.h: ssl_get_new_session and ssl_update_cache.

Change-Id: Ib4b76e2f1983ee066e7806c24721e8626d08a261
Reviewed-on: https://boringssl-review.googlesource.com/1350
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/rand.h b/include/openssl/rand.h
index 62e1037..d17c3ea 100644
--- a/include/openssl/rand.h
+++ b/include/openssl/rand.h
@@ -24,26 +24,26 @@
 
 /* RAND_bytes writes |len| bytes of random data to |buf|. It returns one on
  * success and zero on otherwise. */
-int RAND_bytes(uint8_t *buf, size_t len);
+OPENSSL_EXPORT int RAND_bytes(uint8_t *buf, size_t len);
 
 /* RAND_cleanup frees any resources used by the RNG. This is not safe if other
  * threads might still be calling |RAND_bytes|. */
-void RAND_cleanup();
+OPENSSL_EXPORT void RAND_cleanup();
 
 
 /* Deprecated functions */
 
 /* RAND_pseudo_bytes is a wrapper around |RAND_bytes|. */
-int RAND_pseudo_bytes(uint8_t *buf, size_t len);
+OPENSSL_EXPORT int RAND_pseudo_bytes(uint8_t *buf, size_t len);
 
 /* RAND_seed does nothing. */
-void RAND_seed(const void *buf, int num);
+OPENSSL_EXPORT void RAND_seed(const void *buf, int num);
 
 /* RAND_add does nothing. */
-void RAND_add(const void *buf, int num, double entropy);
+OPENSSL_EXPORT void RAND_add(const void *buf, int num, double entropy);
 
 /* RAND_poll returns one. */
-int RAND_poll(void);
+OPENSSL_EXPORT int RAND_poll(void);
 
 
 #if defined(__cplusplus)