vec_iree: Add the separate device folder for samples

vec_iree/samples/simple_vec_mul/device_embedded_sync.c" is a common file that will be used for
several subdirectories in vec_iree/samples. Thus move it into a newly created and separate directory "device", and build it as a library.

Change-Id: I7a4b981415c5ef6297cc177b18ecf7d1f4387323