tests: coverage: Add Gcov support.

This is a custom Gcov implementation. Taking excerpts from gcc
gcc libgcc/libgcov.h and gcc/gcov-io.h.
Ported to zephyr by Ramakrishna Pallala <ramakrishna.pallala@intel.com>
and Adithya Baglody <adithya.nagaraj.baglody@intel.com>

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>