acpi: add build_append_named_dword, returning an offset in buffer

acpi: add build_append_named_dword, returning an offset in buffer

This is a very limited form of support for runtime patching -
similar in functionality to what we can do with ACPI_EXTRACT
macros in python, but implemented in C.

This is to allow ACPI code direct access to data tables -
which is exactly what DataTableRegion is there for, except
no known windows release so far implements DataTableRegion.

Signed-off-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Xiao Guangrong <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>