SELFOUR-276: Add MCP field to threads.

Where MCP = Maximum Controlled Priority

This commit adds:

* seL4_TCB_SetMCPriority

and changes the arguments to

* seL4_TCB_Configure

As of this commit, a thread cannot create or set a threads
priority (including itself) above its mcp. Previously the kernel
did this check against a threads priority, which prevented a thread
from setting it's own priority down and then up again.

libsel4/tools/syscall_stub_gen.py

@@ -219,6 +219,7 @@ def init_data_types(wordsize):
 
         # seL4 Structures
         BitFieldType("seL4_CapData_t", wordsize, wordsize),
+        BitFieldType("seL4_PrioProps_t", wordsize, wordsize),
 
         # Object types
         CapType("seL4_CPtr", wordsize),