bazel: expand linkstamp beyond binary rules.

Description: linkstamp is currently used by the VersionInfo class to stamp envoy binaries. This is a robust mechanism. However, there are two current problems with it:

1. It is poorly documented and understood. Document git workspace SHA1 inclusion in C++ builds bazelbuild/bazel#2893
2. linkstamp is not available for non-binary rules. Thus if the version_info_lib is compiled outside of a top-level binary rule the linker will not be able to find symbols defined by the linkstamp rule. For now this is resolved by VersionInfo: allow non-linkstamp rules to link in definitions #6803. However, it would be great to use the same, robust linkstamp mechanism we use for binary rules.