XFAIL under Clang tests using labels

gdb/testsuite/
	* gdb.base/label.exp: XFAIL label related tests under Clang.
	* gdb.cp/cplabel.exp: Ditto.
	* gdb.linespec/ls-errs.exp: Refactor tests to execute directly
	and XFAIL under Clang those using labels.