Cast relcount to unsigned long when comparing with sec->reloc_count

Cast relcount to unsigned long when comparing with sec->reloc_count

The type of relcount is long and the type of sec->reloc_count is
unsigned int.  On 32-bit hosts, GCC issues an error:

objcopy.c:2144:20: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
       if (relcount > sec->reloc_count)

Cast relcount to unsigned long to silence GCC.

	* objcopy.c (merge_gnu_build_notes): Cast relcount to unsigned
	long when comparing with sec->reloc_count.