[tsan] Support interception of libdispatch on Linux

This is a new attempt for bringing TSan libdispatch support to Linux.
The main issue with the last patch (https://reviews.llvm.org/D53171) was
that we want to avoid building a separate library.

The updated plan is as follows:
1) Hide libdispatch support behind a flag: true on Darwin, false
   elsewhere. If flag is specified, assume that libdispatch header and
   -flbocks is available for building. This way we can directly include
   the libdispatch header and rely on blocks runtime for our
   implementation.
2) Optionally/weakly intercept libdispatch API functions.

This patch accomplishes 1). It compiles (without the flag enabled) on
Linux. Follow-up patches will provide 2) and enabling of tests on Linux.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D58935

llvm-svn: 355538