Add Makefile to cross-compile from Linux to Amiga

This Makefile can be used to cross-compile pForth on a Linux host to
an Amiga target.  GCC is used as host-compiler and VBCC as
cross-compiler.

* build/linux-crossbuild-amiga/Makefile: New file.
* csrc/pf_io.c, csrc/pf_inner.c: Add some quirks to satisfy VBCC. In
particular fseek and ftell needed some help.