[libc][benchmarks] Link the memory benchmark exes to functions from LLVM libc.

Summary:
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.

Reviewers: gchatelet

Differential Revision: https://reviews.llvm.org/D81910