linux-user: elfload: Provide default for elf_check_arch

For many arch's this macro is defined as the predicatable behaviour
of checking the argument for eqaulity against ELF_ARCH. Provide a
default define as such, so only archs with special handling (usually
allowing multiple EM values) need to provide a def.

Arches that do any of:

1: provide this def exactly the same way as the new default
        (alpha, x86_64)
2: check against ELF_MACHINE while defining ELF_ARCH == ELF_MACHINE
        (arm, aarch64)
3: check against EM_FOO directly while defining ELF_ARCH == EM_FOO
        (unicore32, sparc32, ppc32, mips, openrisc, sh4, cris, m86k)

have their elf_check_arch removed as the default will provide the
correct behaviour.

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>
