arm: Remove ELF_MACHINE from cpu.h

arm: Remove ELF_MACHINE from cpu.h

The only generic code relying on this is linux-user. Linux user
already has a lot of #ifdef TARGET_ customisation so instead, define
ELF_ARCH as either EM_ARM or EM_AARCH64 appropriately.

The armv7m bootloader can just pass EM_ARM directly, as that
is architecture specific code. Note that arm_boot already has its own
logic selecting an arm specific elf machine so this makes V7M more
consistent with arm_boot.

This removes another architecture specific definition from the global
namespace.

Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Acked-By: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>