rs6000, add testcases to the overloaded vec_perm built-in

rs6000, add testcases to the overloaded vec_perm built-in

The overloaded vec_perm built-in supports permuting signed and unsigned
vectors of char, bool char, short int, short bool, int, bool, long long
int, long long bool, int128, float and double.  However, not all of the
supported arguments are included in the test cases.  This patch adds
the missing test cases.

Additionally, in the 128-bit debug print statements the expected result and
the result need to be cast to unsigned long long to print correctly.  The
patch makes this additional change to the print statements.

gcc/ChangeLog:
	* doc/extend.texi: Fix spelling mistake in description of the
	vec_sel built-in.  Add documentation of the 128-bit vec_perm
	instance.

gcc/testsuite/ChangeLog:
	* gcc.target/powerpc/vsx-builtin-3.c: Add vec_perm test cases for
	arguments of type vector signed long long int, long long bool,
	bool, bool short, bool char and pixel, vector unsigned long long
	int, unsigned int, unsigned short int, unsigned char.  Cast
	arguments for debug prints to unsigned long long.
	* gcc.target/powerpc/builtins-4-int128-runnable.c: Add vec_perm
	test cases for signed and unsigned int128 arguments.