samples: syst: Update sample.yaml

Updates the sample.yaml file to exclude problem architectures
from this sample project. SYS-T does not support 64-bit or
big endian architectures. The posix arch is excluded as it
is generating numerous anomolous runtime messages because it
does have the means to determine when data resides in the
rodata section.

It also converts the list of allowed platforms to lists of
platforms to use for integration testing. This helps to increase
general testing coverage.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>