acpi: enable writing ACPI tables on QEMU

Invoke write_acpi_tables() via EVT_LAST_STAGE_INIT on QEMU except on X86.
X86 calls write_acpi_tables() in write_tables().

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>