Improve the definition of STITCHED_CALL in e_rc4_hmac_md5.c

The definition of STITCHED_CALL relies on OPENSSL_NO_ASM.  However,
when a configuration simply lacks the assembler implementation for RC4
(which is where we have implemented the stitched call), OPENSSL_NO_ASM
isn't implemented.  Better, then, to rely on specific macros that
indicated that RC4 (and MD5) are implemented in assembler.

For this to work properly, we must also make sure Configure adds the
definition of RC4_ASM among the C flags.

Reviewed-by: Andy Polyakov <appro@openssl.org>