At the moment, invocation of the 'gcov' tool for coverage analysis
from ccanlint is put directly into the tests_compile_coverage.c and
tests_coverage.c files.  This makes it awkard to extend.

So, this patch moves the invocation of gcov into a new tools/gcov.v
file, analagous to tools/compile.c.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>