Add a "link-guard" to avoid accidentally linking to a wrong dylib at runtime.

We want to prevent compiling something against one version
of a dynamic library and then, at runtime accidentally
using a different version of the dynamic library. With the
old symbol-naming scheme this could not happen because every
symbol had the SVH in it and you'd get an error by the
dynamic linker when using the wrong version of a dylib. With
the new naming scheme this isn't the case any more, so this
patch adds the "link-guard" to prevent this error case.

This is implemented as follows:

- In every crate that we compile, we emit a function called
  "__rustc_link_guard_<crate-name>_<crate-svh>"
- The body of this function contains calls to the
  "__rustc_link_guard" functions of all dependencies.
- An executable contains a call to it's own
  "__rustc_link_guard" function.

As a consequence the "__rustc_link_guard" function call graph
mirrors the crate graph and the dynamic linker will fail if a
wrong dylib is loaded somewhere because its
"__rustc_link_guard" function will contain a different SVH in
its name.

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