gcc/config/mips/mips.h sets it to /lib64:/usr/lib64,
I haven't gotten this far yet, but this seems to be working around multilib. I wonder if it's possible to make 'gcc -mabi=32' when built for mips64 to look in /usr/lib32.
Presumably you would be able to see this in the -print-search-dirs for mips64-gcc. I've been using mips-gcc to build mips64 which is why I haevn't seen this.