gcov: Add coverage supported boards.

Not all boards can get the coverage reports. The report generation
needs minimum of 64KB SRAM. This patch added support for mps2_an385,
qemu_cortex_m3, qemu_x86. This is just a subset. Also the previously
supported board native_posix is included in this patch.

If a board has 64KB RAM, it can generate reports by just
adding HAS_COVERAGE_SUPPORT in Kconfig.board.

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