[libc] revamp memory function benchmark

[libc] revamp memory function benchmark

The benchmarking infrastructure can now run in two modes:
 - Sweep Mode: which generates a ramp of size values (same as before),
 - Distribution Mode: allows the user to select a distribution for the size paramater that is representative from production.

The analysis tool has also been updated to handle both modes.

Differential Revision: https://reviews.llvm.org/D93210