[libc] Annotate RPC members as global pointers (#215412)

Summary:
These are currently all routed through the generic interface. Doing this
should safe a handful of instructions and make the intent clearer.
Basically, makes it nicer on the optimizer and reduces the number of
wait counts while effectively being NFC because this address space is
where these always lived.

Main rationale, lets you link C++ into OpenCL without requiring flat
addressing.
Also trims off some SGPRs I think.

GitOrigin-RevId: 9728186f9c53a4ad1ac115746a0222d39ae6f967