cmake: Support coverage flags on all archs

Most arch's CMakeLists.txt contain rules to add compiler and linker
flags for coverage if CONFIG_COVERAGE is enabled, but 4 of them were
missing this.

Instead, set the coverage flags in arch/common/CMakeLists.txt which
affects all archs.

Signed-off-by: Jeremy Bettis <jbettis@chromium.org>