[POWERPC] Editable kernel command-line in zImage binary.

zImage will set /chosen/bootargs (if it is otherwise empty) with the
contents of a buffer in the section "__builtin_cmdline".  This permits
tools to edit zImage binaries to set the command-line eventually
processed by vmlinux.

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Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>