[PATCH] gdb: define COFF file offsets with file_ptr

[Jameson Nash]

> linetab_size was unsigned, so maybe use ufile_ptr to keep it that way?

It's (currently) computed as the difference of two file_ptr values. The
result should be positive, certainly, as it's later used as either a file
offset (with a printed warning) and as a size_t. But I'm more inclined to
keep it common with the other values here, and use `file_ptr`, which is a
signed integer type like `off_t` and `ssize_t`.