rs6000/test: Fix typo in pr86731-fwrapv-longlong.c [PR106682]

rs6000/test: Fix typo in pr86731-fwrapv-longlong.c [PR106682]

Commit r12-2266 updated the scanned assembly content from

  "{\mlvx\M|\mlxv\M|\mlxvd2x\M}"

to

  "{\mp?lxv\M|\mlxv\M|\mlxvd2x\M}"

for the test case pr86731-fwrapv-longlong.c unexpectedly.

It's meant to update "lxv" to "p?lxv", should leave the
"lvx" unchanged.  So fix the typo accordingly.

	PR testsuite/106682

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr86731-fwrapv-longlong.c: Fix typo.