ld-plugin/lto.exp: Disable ld/12942 test for gcc < 4.7.0.

The test for ld/12942 fails with gcc versions before 4.7.0. This patch
disables this test with these versions of gcc.

ld/testsuite/ChangeLog:

2013-04-29  Will Newton  <will.newton@linaro.org>

	* ld-plugin/lto.exp: Disable ld/12942 test for gcc < 4.7.0.