rs6000: Move g++.dg/ext powerpc tests to g++.target

Also adjust DejaGnu directives, as specifically requiring "powerpc*-*-*" is no
longer required.

2021-02-22  Paul A. Clarke  <pc@us.ibm.com>

gcc/testsuite
	* g++.dg/ext/altivec-1.C: Move to g++.target/powerpc, adjust dg
	directives.
	* g++.dg/ext/altivec-2.C: Likewise.
	* g++.dg/ext/altivec-3.C: Likewise.
	* g++.dg/ext/altivec-4.C: Likewise.
	* g++.dg/ext/altivec-5.C: Likewise.
	* g++.dg/ext/altivec-6.C: Likewise.
	* g++.dg/ext/altivec-7.C: Likewise.
	* g++.dg/ext/altivec-8.C: Likewise.
	* g++.dg/ext/altivec-9.C: Likewise.
	* g++.dg/ext/altivec-10.C: Likewise.
	* g++.dg/ext/altivec-11.C: Likewise.
	* g++.dg/ext/altivec-12.C: Likewise.
	* g++.dg/ext/altivec-13.C: Likewise.
	* g++.dg/ext/altivec-14.C: Likewise.
	* g++.dg/ext/altivec-15.C: Likewise.
	* g++.dg/ext/altivec-16.C: Likewise.
	* g++.dg/ext/altivec-17.C: Likewise.
	* g++.dg/ext/altivec-18.C: Likewise.
	* g++.dg/ext/altivec-cell-1.C: Likewise.
	* g++.dg/ext/altivec-cell-2.C: Likewise.
	* g++.dg/ext/altivec-cell-3.C: Likewise.
	* g++.dg/ext/altivec-cell-4.C: Likewise.
	* g++.dg/ext/altivec-cell-5.C: Likewise.
	* g++.dg/ext/altivec-types-1.C: Likewise.
	* g++.dg/ext/altivec-types-2.C: Likewise.
	* g++.dg/ext/altivec-types-3.C: Likewise.
	* g++.dg/ext/altivec-types-4.C: Likewise.
	* g++.dg/ext/undef-bool-1.C: Likewise.