tests/mem_safe: place test buffers at the edges of RAM

This test checks if it can write at the edges of RAM within the kernel
image. The problem is that this memory is not meant to be trampled on.

With the help of custom linker scripts, place 32-byte buffers at those
edges.

Only linker scripts for QEMU on x86 and Cortex-M3 are provided, to avoid
having to maintain too many of them, in case the reference linker
scripts in the kernel change.

JIRA: ZEP-707

Change-Id: Icd5d680ce2cf064cce083c3d244a196e292bd453
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>