Use is_xcoff_format in ld testsuite

Use is_xcoff_format in ld testsuite

	* testsuite/ld-checks/checks.exp: Use is_xcoff_format.
	* testsuite/ld-powerpc/powerpc.exp: Likewise.
	* testsuite/ld-scripts/print-memory-usage.exp: Likewise.
	* testsuite/ld-srec/srec.exp: Likewise.
	* testsuite/ld-undefined/require-defined.exp: Likewise.
	* testsuite/ld-scripts/expr2.d: Likewise.
	* testsuite/ld-scripts/section-match-1.d: Only run for ELF.
	* testsuite/ld-elfvers/vers.exp: Delete dead code.
	* testsuite/ld-elfvsb/elfvsb.exp: Likewise.
	* testsuite/ld-elfweak/elfweak.exp: Likewise.