[poky] how to specify extra options for toolchain

[张忠山]

I'm using toolchain with newlib, When try to build any library libxml2 for example.
bitbake would build libtool first. But it failed on task libtool-cross_2.4.6.bb:do_configure

I think this may be because the toolchain lack of some startup object file.
So I wan't to let the toolchain using specific .specs file using -specs=file option,

But where to add this? 

It looks like TOOLCAHIN_OPTIONS is  the right place. But TOOLCHAIN_OPTIONS dos not appear 
in any documment.

So where is the right place to add "-specs=file" option

-- 
Best Regards,
zzs