Fix openssl build failure for AARCH64_GCC

[htafr]

Trying to compile with toolchain AARCH64_GCC generates the following error:

$WORKSPACE/libspdm/os_stub/openssllib/openssl/crypto/o_str.c: In function ‘openssl_strerror_r’:
$WORKSPACE/libspdm/os_stub/openssllib/openssl/crypto/o_str.c:329:13: error: implicit declaration of function ‘strerror_r’; did you mean ‘strerror’? [-Werror=implicit-function-declaration]
  329 |     return !strerror_r(errnum, buf, buflen);
      |             ^~~~~~~~~~

Fix: just add the flag -std=c11 in OPENSSL_FLAGS for AARCH64_GCC.

[steven-bellock]

The code in question is

#elif (defined(_POSIX_C_SOURCE) && _POSIX_C_SOURCE >= 200112L) || \
      (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE >= 600)
    /*
     * We can use "real" strerror_r. The OpenSSL version differs in that it
     * gives 1 on success and 0 on failure for consistency with other OpenSSL
     * functions. Real strerror_r does it the other way around
     */
    return !strerror_r(errnum, buf, buflen);