lib/posix-process: Ignore CLONE_VM when assigning tls parameter

lib/posix-process: Ignore CLONE_VM when assigning tls parameter

The interpretation of the tls parameter of clone() is conditional to
passing CLONE_SETTLS in flags. Fix a bug where CLONE_VM would result
into assigning the tls parameter to child. Assigning the parent's tls
when CLONE_VFORK is set, is performed in clone_setup_child_ctx(),
conditionally that the child's tls is not already set, presumably due
to CLONE_SETTLS.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1627