exec: Declare abi_ptr type in its own 'abi_ptr.h' header

exec: Declare abi_ptr type in its own 'abi_ptr.h' header

The abi_ptr type is declared in "exec/cpu_ldst.h" with all
the load/store helpers. Some source files requiring abi_ptr
type don't need the load/store helpers. In order to simplify,
create a new "exec/abi_ptr.h" header.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20231212123401.37493-21-philmd@linaro.org>