testsuite: Fix vec-shift-double-runnable.c for Power10BE [PR127548]

The expected values in vec-shift-double-runnable.c assume
little-endian representation. The vec_sldb and vec_srdb built-ins
have endian-dependent results because the vector operands are
concatenated in big-endian order.

Add big-endian expected values for the vec_sldb and vec_srdb tests
so that the testcase also passes on big-endian.

2026-10-06  Jeevitha Palanisamy  <jeevitha@linux.ibm.com>

gcc/testsuite/
	PR target/127548
	* gcc.target/powerpc/vec-shift-double-runnable.c: Add
	big-endian expected values for vec_sldb and vec_srdb tests.