Rename argument name from link to xlink

Rename argument name from link to xlink

This fixed the build with GCC 4.2:

cc1: warnings being treated as errors
binutils-gdb/binutils/dwarf.c: In function ‘load_separate_debug_info’:
binutils-gdb/binutils/dwarf.c:9650: warning: declaration of ‘link’ shadows a global declaration
/usr/include/unistd.h:757: warning: shadowed declaration is here

	* dwarf.c (load_separate_debug_info): Rename argument name from
	link to xlink.