[libc] Explicitly pin memory for the client symbol lookup (#73988)

Summary:
Previously, we determined that coarse grained memory cannot be used in
the general case. That removed the buffer used to transfer the memory,
however we still had this lookup. Though we do not access the symbol
directly, it still conflicts with the agents apparently. Pin this as
well.

This resolves the problems @lntue was having with the `libc` GPU build.