libpmc: expand PMC_OP* defines

When performing the syscall, specify the full name of the desired
operation. This improves grep-ability.

No functional change.

Reviewed by: jkoshy
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40334

(cherry picked from commit be75e578549fcc1da3754a29fc0e7b63672e7b44)