arch/arm: Fix THREAD_MONITOR entry struct

On ARM, the __thread_entry struct pointer used when
CONFIG_THREAD_MONITOR is enabled was pointing to the initial exception
stack frame.  That's not right: even though this contains all the same
info, it's not layout-compatible with the API struct.  And once the
thread starts running, the ESF gets unwound and the memory recliamed
for the stack frame of the entry function!

Stuff the __thread_entry struct into its own memory at the bottom of
the stack like the other architectures do.  Fixes #7541

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>