cpu: Move cpu_write_elfXX_note() functions to CPUState

Convert cpu_write_elfXX_note() functions to CPUClass methods and pass
CPUState as argument. Update target-i386 accordingly.

Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
[AF: Retain stubs as CPUClass' default method implementation; style changes]
Signed-off-by: Andreas Färber <afaerber@suse.de>