Bignum: Declare loop variable in loop head
Bignum: Declare loop variable in loop head

In the new bignum files (bignum_core.c, bignum_mod_raw.c and
bignum_mod.c) the loop variables are declared in the loop head wherever
this change is beneficial.

There are loops where the loop variable is used after the end of the
loop (this might not be good practice, but that is out of scope for this
commit) and others where there are several loop variables and declaring
them there would hurt readability.

Signed-off-by: Janos Follath <b175c17abe0474719672be1367bb75ee28bd5c71@arm.com>

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