[BOLT][Instrumentation] Put Allocator itslef in shared memory by default

In absence of instrumentation-file-append-pid option,
global allocator uses shared pages for allocation. However, since it is a
global variable, it gets COW'd after fork if instrumentation-sleep-time
is used, or if a process forks by itself. This means it handles the same
pages to every process which causes hash table corruption. Thus, if we
want shared pages, we need to put the allocator itself in a shared page,
which we do in this commit in __bolt_instr_setup.
I also added a couple of assertions to sanity-check the hash table.

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