tile: don't assume user privilege is zero

Technically, user privilege is anything less than kernel
privilege.  We modify the existing user_mode() macro to have
this semantic (and use it in a couple of places it wasn't being
used before), and add an IS_KERNEL_EX1() macro to the assembly
code as well.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>