[fortran,patch] Fix some string length issues

[FX <fxcoudert at gmail dot com>]

>From ancient times, string lengths were assumed to be integers of kind value 4. At some point, we tried to remove that assumption, created a specific type in the library (which happens to be GFC_INTEGER_4, but could be changed) and created gfc_charlen_type_node and gfc_charlen_int_kind (which is defined to be 4, of course).

However, the transition was not complete, and assumptions still lay around in some places. The library seems OK, but the front-end has many issues with this. The way I try is simply to regtest on a build where I redefine character length to be of kind 8. Right now, this hurts.

These are the first things I noticed, which I submit separately for they are pretty obvious.
Bootstrapped and regtested on x86_64-linux. OK to commit?

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