The truth is, noone has managed it to my knowledge.
However, the fact that there is no option si simple to remedy.
in the file extra/Configs/Config.in all you have to do is uncomment the architecture you want.
Huh? What coder wrote above is working fine for me.
You may look into buildroot/Makefile and search those lines:
# Build/install c++ compiler and libstdc++?
#INSTALL_LIBSTDCPP:=true
INSTALL_LIBSTDCPP:=false
Older uClibc (the one coms with the linksys tarball, for example) had lots of problems like incompatible string.h, math lib fucntion names, etc., but these were either fixed or patched in the one that the current buildroot uses. The patches add the mipsel-uclibc target to the gcc/g++/stdlibc++, etc. configs, too.