This avoids the test failure that was introduced in rG32bddad where
this function pulls in the rest of InstrProfilingFile.c which is
undesirable in use cases when profile runtime is being used without
the rest of libc.
This also allows additional cleanup by eliminating another variable
from platforms that don't need it.
Differential Revision: https://reviews.llvm.org/D76750
Petr Hosek