Add -fdefault-integer-8

[meow464]

Question: The passes create a lot of integers and logicals, should those also follow -fdefault-integer-8 or should they be left as 4 bytes wide?

Set the default integer and logical types to an 8 byte wide type. This option also affects the kind of integer constants like 42. Unlike -finteger-4-integer-8, it does not promote variables with explicit kind declaration. 

https://gcc.gnu.org/onlinedocs/gfortran/Fortran-Dialect-Options.html

[certik]

I think the default should be 32bit, for performance and backwards compatibility. And we should add an option to use 64bit by default, like you did in this PR. I think test reference results have to be updated.

[meow464]

I think test reference results have to be updated.

No, my branch had a bug. The references are still good.

[certik]

Thanks! I think this looks good. If there are issues, we can iterate on it with subsequent PRs.