rs6000, extend vec_xxpermdi built-in for __int128 args

rs6000, extend vec_xxpermdi built-in for __int128 args

Add a new signed and unsigned int128 overloaded vector instances for
vec_xxpermdi:

   __int128 vec_xxpermdi (__int128, __int128, const int);
   __uint128 vec_xxpermdi (__uint128, __uint128, const int);

Update the documentation to include a reference to the new vector built-in
instances of vec_xxpermdi.

Add test cases for the new overloaded instances.

gcc/ChangeLog:
	* config/rs6000/rs6000-overload.def (vec_xxpermdi): Add new
	overloaded built-in instances of vector signed and unsigned
	int128.
	* doc/extend.texi: Add documentation for built-in instances of
	vector signed and unsigned int128.

gcc/testsuite/ChangeLog:gcc/testsuite/ChangeLog:
	* gcc.target/powerpc/vec_perm-runnable-i128.c: New test file.