Implement thread_tcb_set() and thread_yield().

The first system call is used to set the user TLS address. Right now
this system call is invoked by the C library for both the initial thread
and additional threads unconditionally, but in the future we'll only
call this if the architecture does not support this. On recent x86-64
CPUs we could use the WRFSBASE instruction.

This system call was erroneously placed in sys/compat/cloudabi64, even
though it does not depend on any pointer size dependent datastructure.
Move it to the right place.

Obtained from:	https://github.com/NuxiNL/freebsd

sys/compat/cloudabi/cloudabi_thread.c

@@ -26,6 +26,10 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#include <sys/param.h>
+#include <sys/proc.h>
+#include <sys/sched.h>
+
 #include <compat/cloudabi/cloudabi_proto.h>
 
 int
@@ -37,11 +41,19 @@ cloudabi_sys_thread_exit(struct thread *td,
 	return (ENOSYS);
 }
 
+int
+cloudabi_sys_thread_tcb_set(struct thread *td,
+    struct cloudabi_sys_thread_tcb_set_args *uap)
+{
+
+	return (cpu_set_user_tls(td, uap->tcb));
+}
+
 int
 cloudabi_sys_thread_yield(struct thread *td,
     struct cloudabi_sys_thread_yield_args *uap)
 {
 
-	/* Not implemented. */
-	return (ENOSYS);
+	sched_relinquish(td);
+	return (0);
 }

sys/compat/cloudabi64/cloudabi64_thread.c

@@ -37,12 +37,3 @@ cloudabi64_sys_thread_create(struct thread *td,
 	/* Not implemented. */
 	return (ENOSYS);
 }
-
-int
-cloudabi64_sys_thread_tcb_set(struct thread *td,
-    struct cloudabi64_sys_thread_tcb_set_args *uap)
-{
-
-	/* Not implemented. */
-	return (ENOSYS);
-}

sys/compat/cloudabi64/syscalls.master

@@ -207,6 +207,5 @@
 					cloudabi64_threadattr_t *attr); }
 54	AUE_NULL	STD	{ void cloudabi_sys_thread_exit( \
 					cloudabi_lock_t *lock); }
-55	AUE_NULL	STD	{ void cloudabi64_sys_thread_tcb_set( \
-					void *tcb); }
+55	AUE_NULL	STD	{ void cloudabi_sys_thread_tcb_set(void *tcb); }
 56	AUE_NULL	STD	{ void cloudabi_sys_thread_yield(); }