[POWERPC] Switch to using user_regset-based core dumps

This switches powerpc to using the user_regset-based code for ELF core
dumps.  The core dumps come out exactly the same either way, except that
the NT_PPC_VMX note is now omitted for any thread that never touched its
Altivec registers (thread_struct.vr_used).
Signed-off-by: NRoland McGrath <roland@redhat.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

include/asm-powerpc/elf.h

@@ -167,6 +167,7 @@ typedef elf_vrreg_t elf_vrregset_t32[ELF_NVRREG32];
 #define elf_check_arch(x) ((x)->e_machine == ELF_ARCH)
 
 #define USE_ELF_CORE_DUMP
+#define CORE_DUMP_USE_REGSET
 #define ELF_EXEC_PAGESIZE	PAGE_SIZE
 
 /* This is the location that an ET_DYN program is loaded if exec'ed.  Typical