dump: select header bitness based on ELF class, not ELF architecture

dump: select header bitness based on ELF class, not ELF architecture

The specific ELF architecture (d_machine) carries Too Much Information
(TM) for deciding between create_header32() and create_header64(), use
"d_class" instead (ELFCLASS32 vs. ELFCLASS64).

This change adapts write_dump_header() to write_elf_loads(), dump_begin()
etc. that also rely on the ELF class of the target for bitness selection.

Considering the current targets that support dumping, cpu_get_dump_info()
works as follows:
- target-s390x/arch_dump.c: (EM_S390, ELFCLASS64) only
- target-ppc/arch_dump.c (EM_PPC64, ELFCLASS64) only
- target-i386/arch_dump.c: sets (EM_X86_64, ELFCLASS64) vs. (EM_386,
  ELFCLASS32) keying off the same Long Mode Active flag.

Hence no observable change.

Approximately-suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>