Log Message:
Add some patches to get us closer to building on NetBSD/powerpc:
 * moz.build: CONFIG['OS_TEST'] is apparently PCU, not MACHINE, so use
   'powerpc' instead of the longish list of powerpc ports.
 * xptcinvoke_asm_ppc_netbsd.s: adapt to use of NS_InvokeByIndex()
 * xptcinvoke_ppc_netbsd.cpp: adapt to use of NS_InvokeByIndex()
 * xptcstubs_ppc_netbsd.cpp: adapt in the direction of xptcstubs_ppc_linux.cpp;
   this has apparently not been build-tested in a while.
The current stumbling block is the lack of 64-bit atomic operations.
No PKGREVISION bump as this is a partial build fix only for NetBSD/powerpc.