CryptoPkg/IntrinsicLib: Fix CLANG38 IA32 build problem

CryptoPkg/IntrinsicLib: Fix CLANG38 IA32 build problem

When use clang-3.8 to build the NetworkPkg, compiler optimization
may use memcpy for memory copy. For example:

 CryptoPkg/Library/OpensslLib/openssl/ssl/ssl_rsa.c:918: undefined
 reference to `memcpy'`

Compiler optimization is sophisticated, but we can work around it
use __attribute__((__used__)) to informs the compiler that symbol
should be retained in the object file, even if it may be
unreferenced.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Signed-off-by: Xiaoyu Lu <xiaoyux.lu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>