stand/ficl 64-bit compatibility

Currently, the only thing that prevents a functioning 64-bit FICL build is
a few integer types that were intended to be fixed-width.

Changing them to C99 integer types allows building a functioning 64-bit
FICL.

While this isn't applicable to the default settings of any in-tree loaders,
it is necessary for a future Petitboot loader, due to the requirement that
it be compiled as a 64-bit program.

Reviewed by: tsoome, imp (earlier revision)
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D26364