[libc] Change RPC opcode enum definition (#67439)

Summary:
This enum previously manually specified the value. This just made it
unnecessarily difficult to add new ones without changing everything.
This patch also makes it compatible with C by removing the `:`
annotation and instead using the `LAST` method.

Author: jhuber6

libc/include/llvm-libc-types/rpc_opcodes_t.h

@@ -9,26 +9,27 @@
 #ifndef __LLVM_LIBC_TYPES_RPC_OPCODE_H__
 #define __LLVM_LIBC_TYPES_RPC_OPCODE_H__
 
-typedef enum : unsigned short {
+typedef enum {
   RPC_NOOP = 0,
-  RPC_EXIT = 1,
-  RPC_WRITE_TO_STDOUT = 2,
-  RPC_WRITE_TO_STDERR = 3,
-  RPC_WRITE_TO_STREAM = 4,
-  RPC_WRITE_TO_STDOUT_NEWLINE = 5,
-  RPC_READ_FROM_STREAM = 6,
-  RPC_OPEN_FILE = 7,
-  RPC_CLOSE_FILE = 8,
-  RPC_MALLOC = 9,
-  RPC_FREE = 10,
-  RPC_HOST_CALL = 11,
-  RPC_ABORT = 12,
-  RPC_FEOF = 13,
-  RPC_FERROR = 14,
-  RPC_CLEARERR = 15,
-  RPC_FSEEK = 16,
-  RPC_FTELL = 17,
-  RPC_FFLUSH = 18,
+  RPC_EXIT,
+  RPC_WRITE_TO_STDOUT,
+  RPC_WRITE_TO_STDERR,
+  RPC_WRITE_TO_STREAM,
+  RPC_WRITE_TO_STDOUT_NEWLINE,
+  RPC_READ_FROM_STREAM,
+  RPC_OPEN_FILE,
+  RPC_CLOSE_FILE,
+  RPC_MALLOC,
+  RPC_FREE,
+  RPC_HOST_CALL,
+  RPC_ABORT,
+  RPC_FEOF,
+  RPC_FERROR,
+  RPC_CLEARERR,
+  RPC_FSEEK,
+  RPC_FTELL,
+  RPC_FFLUSH,
+  RPC_LAST = 0xFFFF,
 } rpc_opcode_t;
 
 #endif // __LLVM_LIBC_TYPES_RPC_OPCODE_H__