cmd/nm, cmd/objdump: fix elf symbol types

cmd/nm, cmd/objdump: fix elf symbol types

Turns out elf.File.Sections is indexed by the actual
section number, not the number minus one.
I don't know why I thought the -1 was necessary.

Fixes objdump test (and therefore build) on ELF systems.

While we're here, fix bounds on gnuDump so that we
don't crash when asked to disassemble outside
the text segment. May fix Windows build or at least
make the failure more interesting.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/92390043