package/dhcpcd: cherry-pick upstream arch-specific privsep fixes

package/dhcpcd: cherry-pick upstream arch-specific privsep fixes

dhcpcd includes privsep-linux.c which contains platform-specific
definitions for the seccomp fixes. A lot of our architectures were not
supported yet in the 9.4.0 release, but are supported now thanks to
Fabrice Fontaine.

Cherry-pick those patches. All of them affect the same code, but they
are cherry-picked individually to keep the correspondence with upstream.
Slight adjustments had to be made but there were no merge conflicts.

Fixes:
 - http://autobuild.buildroot.org/results/9ed863b3ba5e6e0587a48e619395e5bdb7e9c557
 - http://autobuild.buildroot.org/results/affd2f094084c4f53a324830539d07050b83587e
 - http://autobuild.buildroot.org/results/67f39606054930d307ddd0eb7743f06316d41544

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>