libprocstat: try to fix fallout from r361363

The revision caused libprocstat to have two undefined symbols:

• __start_set_pcpu
• __stop_set_pcpu

probably because of __GLOBL() used in sys/pcpu.h under _KERNEL.
The symbols are not accessed by anything and the linker in base does not
complain about them, but some ports are failing to build.
Hack around the problem by providing definitions for those symbols.

Probably there is a better solution, but I could not think of it yet.

Reported by: zeising
MFC after: 3 days
X-MFC with: r361363
Sponsored by: Panzura