[libc] Drop RPC port count and index to 32-bit numbers

The port count and index into the ports was originally written as a
64-bit number. This was with an abundance of caution, however it's
highly unlikely that any configuration will excede a 32-bit number as
most machines  will require something in the low-thousands. Because GPUs
are functionally 32-bit in many of their operations this costs us some
extra time and registers to do the 64-bit operations. Doing this saves
us about four registers in most tests.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D157980