um: restore process name

After the execve() to disable ASLR, comm is now "exe",
which is a bit confusing. Use readlink() to get this
to the right name again.

Disable stack frame size warnings on main.o since it's
part of the initial userspace and can use larger stack.

Fixes: 68b9883c ("um: Discover host_task_size from envp")
Link: https://patch.msgid.link/20241010161411.c576e2aeb3e5.I244d4f34b8a8555ee5bec0e1cf5027bce4cc491b@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>