cmd/ld: fix gcdata and gcbss symbols

cmd/ld: fix gcdata and gcbss symbols

These two symbols don't show up in the Go symbol table
since they're defined in dodata which is called sometime
after symtab. They do, however, show up in the ELF symbol
table.

This regression was introduced in changeset 01c40d533367.

Also, remove the corresponding strings from the ELF strtab
section now that they're unused.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8650043