This fixes a problem with ELF tools thinking they know the
format of the symbol table, as we do not use any of the
standard formats for that table.

This change will probably annoy the Plan 9 users, but I
believe there are other incompatibilities already that mean
they have to use a Go-specific nm.

Fixes #3473.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6500117