Appease clang -Wshadow

The macros BSWAP4 and BSWAP8 have statetemnt expressions
implementations that use local variable names that shadow variables
outside the macro call, generating warnings like this

e_aes_cbc_hmac_sha1.c:263:14: warning: declaration shadows a local variable
      [-Wshadow]
    seqnum = BSWAP8(blocks[0].q[0]);
             ^
../modes/modes_lcl.h:41:29: note: expanded from macro 'BSWAP8'
                            ^
e_aes_cbc_hmac_sha1.c:223:12: note: previous declaration is here
    size_t ret = 0;
           ^

Have clang be quiet by modifying the macro variable names slightly
(suffixing them with an underscore).

Reviewed-by: Rich Salz <rsalz@openssl.org>