Add support for "--enable-code-coverage" option

Add support for "--enable-code-coverage" option

This change adds support for a new option that can be passed to the
configure script: "--enable-code-coverage". Further, the "--enable-gcov"
option has been removed, as this new option provides the same
functionality (plus more).

When using this new option the following make targets are available:

 * check-code-coverage
 * code-coverage-capture
 * code-coverage-clean

Note: these make targets can only be run from the root of the project.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <prakash.surya@delphix.com>
Closes #6670