[libj6] Add thread wrapper class

This new class makes it easier for user programs to spawn threads. This
change also includes support for .hh files in modules, to differentiate
headers that are C++-only in system libraries.

src/libraries/j6/j6.module

@@ -8,6 +8,7 @@ j6 = module("j6",
         "protocol_ids.cpp",
         "syscalls.s.cog",
         "sysconf.cpp.cog",
+        "thread.cpp",
     ],
     public_headers = [
         "j6/cap_flags.h.cog",
@@ -18,6 +19,7 @@ j6 = module("j6",
         "j6/protocols/service_locator.h",
         "j6/syscalls.h.cog",
         "j6/sysconf.h.cog",
+        "j6/thread.hh",
         "j6/types.h",
 
         "j6/tables/log_areas.inc",