[libc][NFC] Cleanup the GPU file I/O utility header (#65680)

Summary:
The GPU uses separate implementations to perform file IO. This is all
done through the RPC interface and we kept it minimal such that we could
treat a `stdin`, `stdout`, or `stderr` handle from the CPU correctly on
the GPU. The RPC implementation uses different opcodes for whether or
not we are using one of the standard streams. This is so we do not need
to initialize anything to access the CPU's standard stream, because the
server knows that it should print to `stdout` if it gets the `STDOUT`
variant of the opcode. It also saves us an RPC call, which are expensive
relatively  speaking. This patch simply cleans up this interface to make
them all use a common function. This is done in preparation to implement
some more file IO functions like getc or putc.