I chrooted into a failed glibc mockbuild and ran "make" again in the glibc build directory to get the gcc incantation for libanl.so that has been causing the glibc build to fail. The beginning of the several-lines-long gcc command looks like:
gcc -shared -static-libgcc -Wl,-O1 -Wl,-z,defs -Wl,-dynamic-linker=/lib/ld-linux.so.3
If you remove the "-static-libgcc" flag from the gcc command, libanl.so is able to build successfully. I don't know why libgcc_eh.a's __stack_chk_guard isn't resolved by the definition in /lib/ld-linux.so.3, but the problem can apparently be sidestepped by removing "-static-libgcc" from the equation entirely.
I figured I'd share my findings in case someone knows how to get around this from within the makefiles. I'm still hacking at it, but I haven't gotten anywhere yet.