CryptoPkg: Fix possible build problem with Clang

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1089

When building CryptoPkg with Clang, blow error may result

* /usr/lib/llvm-3.8/lib/clang/3.8.0/include/stdatomic.h:105:17: error:
    unknown type name 'wchar_t'

Since the C native atomics are C11 feature we can explicitly
use C99 to work around it.
add -std=c99 to avoid it

* openssl/crypto/conf/conf_sap.c:71:12: error: variable 'ret' is
  uninitialized
  when used here [-Werror,-Wuninitialized]

Suppress warnings in OpenSSL so we don't break the build with -Werror.
add -Wno-error=uninitialized to disalbe this warning

Cc: Ting Ye <ting.ye@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Signed-off-by: Xiaoyu Lu <xiaoyux.lu@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>

CryptoPkg/Library/BaseCryptLib/PeiCryptLib.inf

@@ -93,3 +93,5 @@
   # -JCryptoPkg/Include : To disable the use of the system includes provided by RVCT
   # --diag_remark=1     : Reduce severity of "#1-D: last line of file ends without a newline"
   RVCT:*_*_ARM_CC_FLAGS = -JCryptoPkg/Include --diag_remark=1
+
+  GCC:*_CLANG38_*_CC_FLAGS = -std=c99