cmake: Improved hinting for gmock.

- Removed mandatory reliance on s_gmock_root.
- s_gmock_root is only defaulted if not set AND
s_third_party_common_source_root is.
plat-link-test: Omitted if gmock is not found.
cmake: Added small comment.
cmake: Quoting is now consistent.
Moved axiom after 3rd party in build.md.
cmake: Bashed stale chunk.
cmake: Moved gmock subproject into find module.
plat-link-test now always runs after build.

This should have the same behavior as a unit test. It will automatically
run either when building natively or when an emu is avail, and automatic
running isn't explicitly disabled.

For now, it just always runs.