[PATCH] Introduce SANITIZER_HAS_WEAK_SYMBOL and use it for SymbolizerPrepareForSandboxing.

[Peter Collingbourne]

  OK.  The reason why I introduced the weak symbol is that SymbolizerPrepareForSandboxing on Linux is in the libc-dependent part of sanitizer_common, but I think it can be easily moved into the libc-independent part.

http://llvm-reviews.chandlerc.com/D877