cmd/ld: include full symbol table in Mach-O output

This makes binaries work with OS X nm.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7558044
cmd/ld: wrap long data lines in -a output

Also move symbol names onto lines by themselves: some are very long.
Show relocations.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7689043