[libc] Check the RPC server once again after the kernel exits

We support asynchronous sends, that means that the kernel can issue a
send, then exit the kernel as we do with the `EXIT` syscall. Because of
the condition it's therefore possible for the kernel to exit and break
from the loop before we check the server again. This can potentially
cause us to ignore an `EXIT` call from the GPU.

Reviewed By: JonChesterfield, lntue

Differential Revision: https://reviews.llvm.org/D150456