Mount /dev/shm in chroots

Fix configure crash when building mozjs60 by mounting tmpfs as /dev/shm
in chroots. This is an important fix, because of this dependency chain,
that is currently broken in armhf and armv7 (see pmaports#244):

postmarketos-base -> networkmanager -> polkit -> mozjs60

pmb/chroot/mount.py

@@ -83,6 +83,9 @@ def mount_dev_tmpfs(args, suffix="native"):
 
     # Create pts, shm folders and device nodes
     pmb.helpers.run.root(args, ["mkdir", "-p", dev + "/pts", dev + "/shm"])
+    pmb.helpers.run.root(args, ["mount", "-t", "tmpfs",
+                                "-o", "nodev,nosuid,noexec",
+                                "tmpfs", dev + "/shm"])
     create_device_nodes(args, suffix)