binutils: create PLT entries for unknown symbols

Previously, we were failing to create them unless they were known to
be STT_FUNC.  But if they have yet to be seen, they're STT_NOTYPE.
binutils, glibc: remove unused reloc

Also, clean up dl-machine.h a bit.
gcc: improve PIC code generation

First of all, never spill symbolic addresses to the constant pool.  This
creates costly dynamic relocations while providing little benefit.

Second, due to a bug, we were failing to elide the GOT for local symbol
references with nonzero offsets.