Q:

How could Clang release under BSD license?

I found that llvm could only generate assembly code. If someone want generate and link assembly code to native executable code, they should use external tool(GNU as/ld, for example. Currently lld is not ready, and there no more information about llvm-mc). 
This question's answer and other answers imply that Clang use Binutils(as and ld) to generate and link native code(I'm not quite sure about this). If so, how could Clang release  under BSD like license? I can't find any license exception(like Bison Exception) about ld or as.
If I misunderstand some concepts, please forgive me! BTW, could we use GNU as and ld in programmatically way?
EDIT: Thanks for these answers. It seems that clang still not a standalone toolchain and have to use VS or GNU tools. Does it mean if someone write a compiler based on llvm. If users didn't install GNU or VS tools. The compiler is not a out-of-the-box compiler?

A:

AFAIK (though I am not a lawyer), running a GPL'd application from the command line or similar does not count as derivative work.
Aside from that, the LLD subproject is creating a similarly-licensed linker. Although I don't think it's used by Clang yet.

