Make sure build_SYS_str_reasons() preserves errno

This function can end up being called during ERR_get_error() if we are
initialising. ERR_get_error() must preserve errno since it gets called via
SSL_get_error(). If that function returns SSL_ERROR_SYSCALL then you are
supposed to inspect errno.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7680)