My current toolchain on my build machine is the latest one, compiled
   by a not-quite-so-latest one. I am not[sic] trying to recompile it again, and
   I'm getting:
   
   .../gcc/xgcc -B/usr1/netbsd/build/branch/toolchain5/gcc/  -DIN_GCC   -g -I./
  include  enquire.o -o enquire
   .../gcc/ld: internal error /usr1/netbsd/co/branch/toolchain/ld/ldexp.c 457
   
   Any thoughts?

Says here you hit the default case in fold_name().  I'd guess either you're
trying to link something with a new type of node in the tree, or something
elsewhere corrupted the tree data.  Is this toolchain source something we
can grab so as to compare notes with you?  The last toolchain file put out
was a binary only...