libpmc: Move libpmc and utils to a new pmc package

Due to the size of libpmc.so.5, this reduces the size of the -utilities
package by 10%.

MFC after: 1 day
Reviewed by: manu, adrian, emaste
Differential Revision: https://reviews.freebsd.org/D52662

(cherry picked from commit 67c3c3a274468d09925af3cf64da3f9cf878cc80)