sys/pcpu.h: remove a workaround for a fixed ld bug
sys/pcpu.h: remove a workaround for a fixed ld bug

The workaround was incorrectly documented as having something to do with
set_pcpu section's progbits, but in fact it was for incorrect placement
of __start_set_pcpu because of the bug in ld.
The bug was fixed in r210245, see commit message for details.

A side-effect of the workaround was that a zero-size set_pcpu section was
produced for modules, source code of which included pcpu.h but didn't
actually define any dynamic per-cpu variables.
This commit should remove the side-effect.

The same workaround is present sys/net/vnet.h, has an analogous side-effect
and can be removed as well.

An UPDATING entry that warns about a need for recent ld is following.

MFC after:	1 month

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