General definition for weak functions

In this diff, I define a general macro for defining weak functions
with a default implementation: "SANITIZER_INTERFACE_WEAK_DEF()".
This way, we simplify the implementation for different platforms.

For example, we cannot define weak functions on Windows, but we can
use linker pragmas to create an alias to a default implementation.
All of these implementation details are hidden in the new macro.

Also, as I modify the name for exported weak symbols on Windows, I
needed to temporarily disable "dll_host" test for asan, which checks
the list of functions included in asan_win_dll_thunk.

Differential Revision: https://reviews.llvm.org/D28596

llvm-svn: 293419