coff/pe/xcoff and --extract-symbols

This fixes failure of the "extract symbols" test for rs6000, where
--extract-symbols generates a non-zero sized .text.  By the look of
coffcode.h the same problem might occur for coff/pe too, but doesn't
happen to trigger a test failure.

bfd/
	* coffcode.h (coff_compute_section_file_positions): Don't
	adjust size of !SEC_LOAD sections.
binutils/
	* objcopy.c (setup_section): Clear SEC_LOAD for --extract-symbol.