powerpc: Fix random memory corruption in merged elf.h

The merged verison of ELF_CORE_COPY_REGS is basically the PPC64 version, with
a memset that came from PPC and a few types abstracted out into #defines. But
it's not _quite_ right.

The first problem is we calculate the number of registers with:
        nregs = sizeof(struct pt_regs) / sizeof(ELF_GREG_TYPE)

For a 32-bit process on a 64-bit kernel that's bogus because the registers are
64 bits, but ELF_GREG_TYPE is u32, so nregs == 88 which is wrong.

The other problem is the memset, which assumes a struct pt_regs is smaller
than a struct elf_regs. For a 32-bit process on a 64-bit kernel that's false.

The fix is to calculate the number of regs using sizeof(unsigned long), which
should always be right, and just memset the whole damn thing _before_ copying
the registers in.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>