[libc][rpc] Allocate a single block of shared memory instead of three

[libc][rpc] Allocate a single block of shared memory instead of three

Allows moving the pointer swap between server and client into reset.
Single allocation simplifies whatever allocates the client/server, currently
the libc loaders.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D150337