Use separate counters for 8-fold and single multiplication steps
Use separate counters for 8-fold and single multiplication steps

Compilers are likely to generate shorter assembly for loops of the
form `while( cnt-- ) { ... }` rather than
`for( ; count >= X; count -= X ) { ... }`. (E.g. the latter needs
a subtract+compare+branch after each loop, while the former only
needs decrement+branch).

Signed-off-by: Hanno Becker <4d488caff78d6184f59bbe6e137e34d4606b9fe8@arm.com>

ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls