Enable dlltool --leading-underscore for targets other than x86

This also makes the dlltool tests run more PE targets, finding that
sh-pe dlltool reports "Machine 'sh' not supported".  I guess no one
cares about that.

	PR19459
	* dlltool.c (asm_prefix): Remove "mach" parameter.  Return
	leading_underscore independent of machine.
	(ASM_PREFIX): Adjust.
	* testsuite/binutils-all/dlltool.exp: Run on any target
	satisfying is_pecoff_format for which dlltool is built.
	Revert commit 0398b8d6c86a.  Remove target_xfail.