arch: arm: aarch32: Fix when mode offset is defined

Commit a2cfb843 ("arch: arm: Add code for swapping threads between
secure and non-secure") changed the mode variable in the _thread_arch to
be defined by ARM_STORE_EXC_RETURN or USERSPACE.  The generated offset
define for mode was enabled by FPU_SHARING or USERSPACE.  This broke
Cortex-R with FPU, but with ARM_STORE_EXC_RETURN disabled.  Reconcile
the checks.

Signed-off-by: Bradley Bolen <bbolen@lexmark.com>