ztress: Coherence fix for tests/lib/spsc_pbuf

When CONFIG_KERNEL_COHERENCE=y, it's illegal to place shared
data(like ztress_context_data in this test case) on the stack,
because that memory is incoherent with respect to other CPUs
in the system.

In this specific case, it will cause PC register to load invalid
handler function pointer and crash the Xtensa processor.

Make it static.

Signed-off-by: Ming Shao <ming.shao@intel.com>