testsuite: change gcc.target/powerpc builtins lp64 to int128 requirement.

testsuite: change gcc.target/powerpc builtins lp64 to int128 requirement.

Some of the builtins* tests check for lp64 as a proxy for int128 support.
This patch changes the requirements to int128.  It also removes
some redundant requirements from revb.

gcc/testsuite/ChangeLog

2020-06-30  David Edelsohn  <dje.gcc@gmail.com>

	* gcc.target/powerpc/builtins-2-p9-runnable.c: lp64 to int128.
	* gcc.target/powerpc/builtins-6-p9-runnable.c: Same.
	* gcc.target/powerpc/builtins-6-runnable.c: Same.
	* gcc.target/powerpc/builtins-revb-runnable.c: Same.