Fortran: Detect pointer size on Intel archs with PGI (#14870)

The PGI compiler does not define __SIZEOF_POINTER__ so use the
__x86_64__ or __i386__ CPU macros to select a pointer size instead.
Merge topic 'Fortran-pgi-intel-arch' into next

eb8cb2c6 Fortran: Detect pointer size on Intel archs with PGI (#14870)