ld: Run PR ld/34570 tests only if supported

Since Clang doesn't support all GCC options, PR ld/34570 tests fail to
compile if GCC specific options are used to build binutils.  Add
llvm_lto_tests to lto.exp and don't run PR ld/34570 tests if Clang fails
to compile them.

	PR ld/34576
	PR ld/34570
	* testsuite/ld-plugin/lto.exp (llvm_lto_tests): New.
	Use it.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>