build: container: Use the same clone flags as fork(3).

The intent is to make 'clone' behave a lot more like 'primitive-fork', which
calls clone(2) with SIGCHLD, CLONE_CHILD_CLEARTID, and CLONE_CHILD_SETTID
flags.  Notably, running 'clone' at the REPL without these flags would break
the REPL beyond repair.

* guix/build/syscalls.scm (CLONE_CHILD_CLEARTID, CLONE_CHILD_SETTID): New
  variables.
* gnu/build/linux-container.scm (namespaces->bit-mask): Add
  CLONE_CHILD_CLEARTID and CLONE_CHILD_SETTID to bit mask.

gnu/build/linux-container.scm

@@ -162,7 +162,8 @@ host user identifiers to map into the user namespace."
 (define (namespaces->bit-mask namespaces)
   "Return the number suitable for the 'flags' argument of 'clone' that
 corresponds to the symbols in NAMESPACES."
-  (apply logior SIGCHLD
+  ;; Use the same flags as fork(3) in addition to the namespace flags.
+  (apply logior SIGCHLD CLONE_CHILD_CLEARTID CLONE_CHILD_SETTID
          (map (match-lambda
                ('mnt  CLONE_NEWNS)
                ('uts  CLONE_NEWUTS)

guix/build/syscalls.scm

@@ -50,6 +50,8 @@
             mkdtemp!
             pivot-root
 
+            CLONE_CHILD_CLEARTID
+            CLONE_CHILD_SETTID
             CLONE_NEWNS
             CLONE_NEWUTS
             CLONE_NEWIPC
@@ -303,12 +305,14 @@ string TMPL and return its file name.  TMPL must end with 'XXXXXX'."
         (pointer->string result)))))
 
 ;; Linux clone flags, from linux/sched.h
-(define CLONE_NEWNS   #x00020000)
-(define CLONE_NEWUTS  #x04000000)
-(define CLONE_NEWIPC  #x08000000)
-(define CLONE_NEWUSER #x10000000)
-(define CLONE_NEWPID  #x20000000)
-(define CLONE_NEWNET  #x40000000)
+(define CLONE_CHILD_CLEARTID #x00200000)
+(define CLONE_CHILD_SETTID   #x01000000)
+(define CLONE_NEWNS          #x00020000)
+(define CLONE_NEWUTS         #x04000000)
+(define CLONE_NEWIPC         #x08000000)
+(define CLONE_NEWUSER        #x10000000)
+(define CLONE_NEWPID         #x20000000)
+(define CLONE_NEWNET         #x40000000)
 
 ;; The libc interface to sys_clone is not useful for Scheme programs, so the
 ;; low-level system call is wrapped instead.