Parallelize benchmarking

[boomanaiden154]

With the large scale of our datasets (potentially 10^8 BBs), we will need a reasonably fast way to benchmark basic blocks. Parallelizing this is an obvious first step. This needs a couple things implemented on the LLVM side:

• Shared memory names (used for memory annotations) need a name that is also based on the thread ID rather than just the process ID.
• There needs to be an option to pin a benchmarking process to a specific core within llvm-exegesis.

(There might be more on the llvm-exegesis side).

Then, we need to do the following:

• Implement parallel benchmarking using LLVM threading primitives.
• Validate that running on multiple threads doesn't impact results (using validation counters).
• Ship it.

[boomanaiden154]

Doing this outside of the process with ray seems to work much better.