dtrace: Fix the definition of pc_t

This type is used only to store PC values corresponding to a thread
stack trace, so a pointer type is not quite right. Switch to
vm_offset_t, as in struct stack, to simplify a port of DTrace to CHERI.

No functional change intended.

MFC after: 1 week
Sponsored by: Innovate UK

(cherry picked from commit 8ce6b4f2c48eca758fac90b58924f9b2e38fbc49)