regen/mph.pl: Use 'bignum'

regen/mph.pl: Use 'bignum'

Commit 72196ef94b98987bb277d8bf6db6efaacd624c3c changed to 'use integer'
in one function.  But this was not sufficient to work on a 32 bit
system.  I tried a global 'use integer', which also didn't work; then a
global 'use bigint', which causes the program to not loop and it
generates usable data, but this causes a floating point statistic that is
included to be printed as 0.  The solution is to 'use bignum' instead.

See the thread beginning at
http://nntp.perl.org/group/perl.perl5.porters/251393