gdb: define COFF file offsets with file_ptr

gdb: define COFF file offsets with file_ptr

The arguments to these functions are file_ptr, so these declarations
were accidentally implicitly down-casting them to signed int. This
allows for reading files between 2 and 4 GB in size in my testing (I
don't have a larger dll currently to test). These may not be natively
supported by Windows, but can appear when using split-dwarf information.

This solves a "can't get string table" error resulting from attempting
to pass a negative offset to bfd_seek. I encountered this occuring while
trying to use a debug file for libLLVM.dll, but searching online reveals
at least one other person may have run into a similar problem with
Firefox?

    https://sourceforge.net/p/mingw-w64/mailman/mingw-w64-public/thread/CA+cU71k2bU0azQxjy4-77ynQj1O+TKmgtaTKe59n7Bjub1y7Tg@mail.gmail.com/

With this patch, the debug file appears to load successfully and I can
see debug information in gdb for the library.

gdb/ChangeLog:

	* coffread.c (linetab_offset): Change type to file_ptr.
	(linetab_size): Likewise.
	(enter_linenos): Change parameter type to file_ptr.
	(init_lineno): Likewise.
	(init_stringtab): Likewise.
	(coff_symtab_read): Likewise.
	(coff_symfile_read): Change variable types to file_ptr.

Change-Id: I6ae3bf31efc51c826734ade6731ea6b1c32129f3