[libc] Export a pointer to the RPC client directly (#117913)

Summary:
We currently have an unnecessary level of indirection when initializing
the RPC client. This is a holdover from when the RPC client was not
trivially copyable and simply makes it more complicated. Here we use the
`asm` syntax to give the C++ variable a valid name so that we can just
copy to it directly.

Another advantage to this, is that if users want to piggy-back on the
same RPC interface they need only declare theirs as extern with the same
symbol name, or make it weak to optionally use it if LIBC isn't
avaialb.e

GitOrigin-RevId: 89d8e70031189eacb915beae2ffc642c0de1ec1a
4 files changed