Rewrite add_sysroot

Rewrite add_sysroot

Relative paths shouldn't have the sysroot prefix added.  The patch
also makes some attempt at supporting DOS paths, and tidies code using
the new add_sysroot.

	* emultempl/elf32.em (gld${EMULATION_NAME}_add_sysroot): Rewrite.
	Only prefix absolute paths with sysroot.  Handle DOS paths.
	(gld${EMULATION_NAME}_check_ld_elf_hints): Constify variable.
	(gld${EMULATION_NAME}_check_ld_so_conf): Likewise.
	(gld${EMULATION_NAME}_after_open): Short-circuit NULL path
	searches.  Rename variable.  Simplify get_runpath search.