Add support for LLVM objdump.

Set base address on OSX.

This fixes issue #311. The mapped address of libraries was never considered, nor was the load address of the segments. It also fixes a couple of issues in binutils.go, such as when the tail of the data wasn't handled right when grouping symbols at the same address.

Mainly use imported structs by field and update a mismatch printf
string on a test error message.

Disassembly reports generated by pprof -disasm will now include line number
information as generated by objdump. This will make the generated assembly more
readable.

As part of this I've introduced a new assemblyInstruction struct. Previously
the code was reusing the graph.Node to represent assembly instructions but it
seems better to have a dedicated type for this.

Instead of including it the demangle package with pprof,
point to its own repo.

Provide native Go mocks for binutils to use during testing, instead of
relying on shell-script mocks of those tools.

Also update the build instructions to use go get