Check cpu_softc is not NULL before dereferencing

In the acpi_cpu_postattach SYSINIT function cpu_softc may be NULL, e.g.
on arm64 when booting from FDT. Check it is not NULL at the start of
the function so we don't try to dereference a NULL pointer.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 4e50efb1944bffe6ae648c8c81bd0814c18474b9)