configure: unify x86_64 and x32

The only difference between the two, as far as either configure or
Meson are concerned, is in the multilib flags passed to the compiler.

For QEMU, this fixes the handling of TYPE_OLDDEVT in
include/exec/user/thunk.h and enables testing of dirty ring buffer,
because both are using HOST_X86_64.

For tests/tcg, this means that on a hypothetical x32 host the
cross compiler will not be used to build the tests.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>