[libc][benchmarks] Link the memory benchmark exes to functions from LLVM libc.
To get the target order correct, the benchmarks directory has been moved
one level higher. Previously, it was living in the utils directory. The
utils directory is a collection of utils which are to be used by the
tests and implementations. However, benchmarks *use* the
implementations. So, moving it out of utils helps us setup proper
target level dependencies.
Differential Revision: https://reviews.llvm.org/D81910