DWARF version is clamped to at most 3

[alexcrichton]

Looks like LLVM is capabable of and is currently emitting dwarf version 4 (after upgrading LLVM to the current master). Our bots don't have a gdb which understands this version of dwarf (they're all on gdb 7.4), but gdb 7.6 does indeed understand this version.

For now I'm going to clamp the emitted version to 3 for all platforms but mac (which is on version 2), but we should in theory do something more graceful.

cc @michaelwoerister
cc #13513

[michaelwoerister]

I don't think that this will be an actual issue anytime soon, as we don't use anything DWARF 4 specific, as far as I know.

[thestinger]

I think DWARF 4 makes the debug information smaller, so it would be nice to get the bots upgraded to use it eventually.