cskyelf.em branch stub handling

cskyelf.em branch stub handling

This fixes the csky-elf ld-elf/pr21884 and ld-unique/pr21529 failures,
by disabling branch stubs for binary (and other non-csky) output.
The csky-linux target gets branch stubs off by default because
presumably there are reasons why branch stubs were disabled, but
rather than killing the support completely it can be enabled by
--branch-stub.

	* emultempl/cskyelf.em (csk_elf_before_parse): New function,
	setting use_branch_stub false for linux.
	(csky_elf_create_output_section_statements): Do emit this
	function and all others in the file for linux, plus the branch
	option control.  Disable branch stubs when non-ELF.