[BOLT][Instrumentation] Don't share counters when using append-pid

The point of append-pid option is to record separate profiles for
separate forks, which is impossible when counters are the same for
every process. It leads to a sum of all profiles in every file, plus
GlobalWriteProfileMutex located in a shared memory prevents some
processes from dumping their data at all.

Reviewed By: rafauler, Amir
Differential Revision: https://reviews.llvm.org/D153771