rs6000: Move more g++.dg powerpc tests to g++.target

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

2022-04-15  Paul A. Clarke  <pc@us.ibm.com>

gcc/testsuite
	* g++.dg/debug/dwarf2/const2.C: Move to g++.target/powerpc.
	* g++.dg/other/darwin-minversion-1.C: Likewise.
	* g++.dg/eh/ppc64-sighandle-cr.C: Likewise.
	* g++.dg/eh/simd-5.C: Likewise.
	* g++.dg/eh/simd-4.C: Move to g++.target/powerpc, adjust dg directives.
	* g++.dg/eh/uncaught3.C: Likewise.
	* g++.dg/other/spu2vmx-1.C: Likewise.