armv*-mont.pl: Correct a carry flag comment

armv*-mont.pl: Correct a carry flag comment

On Arm, C is a true carry flag, unlike x86 where CF is a borrow flag for
subtraction. That is, for subtraction, it is set if Rn + ~Rm + 1 carries
in unsigned arithmetic, and so for subtracting 0, i.e. adding ~0 + 1, it
always sets the carry flag. Correct the comment to document what this is
actually doing.

CLA: trivial

Fixes: d1671f4f1a39 ("bn/asm/armv4-mont.pl: add NEON code path.")
Fixes: 10646160125a ("Optimize RSA on armv8")
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27826)