add gcov support

[khale]

It would be great to have gcov output for generating test coverage results and for general informational purpose. Supporting this in a running kernel will require some output channel over which gcov report files can be generated. Generally, I can see the following path for this:

• add -coverage flag to the flags for clang and gcc when we wan't to generate code coverage support
• add a build option in Kconfig for using gcov
• add runtime hooks for gcov (see here and search for gcov in the linux source tree (e.g. kernel/gcov/base.c))
• output gcov data over the qemu debug device using the debugcon=xxx flag
• build a tool to parse and separate the output from QEMU into gcov files
• integrate gcov output into CI tool (.travis.yaml)