Fix typo in fold-vec-load-builtin_vec_xl-* tests.

Fix typo in fold-vec-load-builtin_vec_xl-* tests.

When I checked in the fix for running tests on power10 systems with
power10 code generation, I had a typo in the
fold-vec-load-builtin_vec_xl-* tests, swapping 'x' and 'v' in the p?lxv
pattern.

gcc/testsuite/
2021-08-12  Michael Meissner  <meissner@linux.ibm.com>

	* gcc.target/powerpc/fold-vec-load-builtin_vec_xl-char.c: Fix
	typo in regular expression.
	* gcc.target/powerpc/fold-vec-load-builtin_vec_xl-double.c:
	Likewise.
	* gcc.target/powerpc/fold-vec-load-builtin_vec_xl-float.c:
	Likewise.
	* gcc.target/powerpc/fold-vec-load-builtin_vec_xl-int.c:
	Likewise.
	* gcc.target/powerpc/fold-vec-load-builtin_vec_xl-longlong.c:
	Likewise.
	* gcc.target/powerpc/fold-vec-load-builtin_vec_xl-short.c:
	Likewise.