Move the more readable version of bn_mul_mont() from bn_asm.c to bn_mont.c.

Move the more readable version of bn_mul_mont() from bn_asm.c to bn_mont.c.

Nothing actually uses this code, as OPENSSL_BN_ASM_MONT is not defined
unless there is an assembly implementation available (not to mention that
defining both OPENSSL_NO_ASM and OPENSSL_BN_ASM_MONT at the same time is
extra strange).

Discussed with tb@