ld test asciz and ascii fails

ld test asciz and ascii fails

Fix these fails:
alpha-dec-vms  +FAIL: ld-scripts/asciz
alpha-dec-vms  +FAIL: ld-scripts/ascii
i386-go32  +FAIL: ld-scripts/asciz
sh-coff  +FAIL: ld-scripts/asciz

It's better to positively select targets for .section support than to
try to exclude all targets that don't.  Make a new is_coff_format so
we can easily select such.

binutils/
	* testsuite/lib/binutils-common.exp (is_coff_format): New.
ld/
	* testsuite/ld-scripts/ascii.d: Use is_elf_format and
	is_coff_format to select targets, exclude ti coff.
	* testsuite/ld-scripts/asciz.d: Likewise.  Accept trailing zeros.