Update binary_get_section_contents to seek using section's file position

Update binary_get_section_contents to seek using section's file position

I have a patch for GDB which opens and reads from BFDs using the
"binary" target.  However, for it to work, we need to be able to get a
section's contents based from the file position of that section.

At the moment, reading a section's contents will always read from the
start of the file regardless of where that section is located.  While
this was fine for the original use of the "binary" target, it won't
work for my use case.  This change shouldn't impact any existing
callers due to the fact that the single .data section is initialized
with a filepos of 0.

bfd/ChangeLog:

	* binary.c (binary_get_section_contents): Seek using offset
	from section's file position.