LLVM tools, particularly lldb and dsymutil, don't support base address
selection entries in location lists. When targeting GOOS=darwin,
mode, have the linker translate location lists to CU-relative form
instead.

Technically, this isn't necessary when linking internally, as long as
nobody plans to use anything other than Delve to look at the DWARF. But
someone might want to use lldb, and it's really confusing when dwarfdump
shows gibberish for the location entries. The performance cost isn't
noticeable, so enable it even for internal linking.

Doing this in the linker is a little weird, but it was more expensive in
the compiler, probably because the compiler is much more stressful to
the GC. Also, if we decide to only do it for external linking, the
compiler can't see the link mode.

Benchmark before and after this commit on Mac with -dwarflocationlists=1:

name        old time/op       new time/op       delta
StdCmd            21.3s ± 1%        21.3s ± 1%    ~     (p=0.310 n=27+27)

Only StdCmd is relevant, because only StdCmd runs the linker. Whatever
the cost is here, it's not very large.

Change-Id: I200246dedaee4f824966f7551ac95f8d7123d3b1
Reviewed-on: https://go-review.googlesource.com/89535Reviewed-by: David Chase <drchase@google.com>