testsuite: update builtins-5-p9-runnable.c for BE

testsuite: update builtins-5-p9-runnable.c for BE

Hi,

As PR108809 mentioned, vec_xl_len_r and vec_xst_len_r are tested
in gcc.target/powerpc/builtins-5-p9-runnable.c.
The vector operand of these two bifs are different from the view
of v16_int8 between BE and LE, even it is same from the view of
128bits(uint128/V1TI).

The test case gcc.target/powerpc/builtins-5-p9-runnable.c was
written for LE environment, this patch updates it for BE.

Tested on ppc64 BE and LE.
Is this ok for trunk?

BR,
Jeff (Jiufu)

gcc/testsuite/ChangeLog:

	PR testsuite/108809
	* gcc.target/powerpc/builtins-5-p9-runnable.c: Update for BE.