testsuite: Adjust fold-vec-extract-double.p9.c for powerpc BE [PR108810]

On BE, the extracted index for the leftmost element is 0
rather than 1, adjust the test case accordingly.

	PR testsuite/108810

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/fold-vec-extract-double.p9.c (testd_cst): Adjust
	the extracted index for BE.