testsuite: Gate gcc.dg/plugin/must-tail-call-1.c and -2.c on tail_call

While gcc.dg/plugin/must-tail-call-2.c passes for all targets even
without this, the error message is, for a target like cris-elf that
doesn't implement sibling calls: "error: cannot tail-call: machine
description does not have a sibcall_epilogue instruction pattern"
rather than "error: cannot tail-call: callee returns a structure".
Also, it'd be confusing to exclude must-tail-call-1.c but not
must-tail-call-2.c

	* gcc.dg/plugin/must-tail-call-1.c, gcc.dg/plugin/must-tail-call-2.c:
	Gate on effective target tail_call.