And merge the two out-of-sync copies for adding gcc target flags. The flag
was omitted for gcc as linker already, but not for gcc as external assembler.
This PR is only meant to change Darwin link line with -fsanitize=address. With this PR, I can use addresssanitizer successfully (and easily) on my Mac.
The PR finds a libldc_rt.asan_osx_dynamic.dylib or libclang_rt.asan_osx_dynamic.dylib in LDC's lib dir, and falls back to the old behavior (passing -fsanitize=address to the linker driver) if nothing is found. So now the user can copy his self-built library to LDC's lib dir and be happy :-)
(the cmake script could copy+rename the library to the lib dir when the library is found in the LLVM binaries)
