Dsymutil, an utility used on macOS when externally linking executables,
does not support base address selector entries in debug_ranges.

To work around this deficiency this commit removes base address
selectors from debug_ranges and emits instead a list composed only of
compile unit relative addresses.

A new type of relocation is introduced, R_ADDRCUOFF, similar to
R_ADDROFF, that relocates an address to its offset from the low_pc of
the symbol's compile unit.

Fixes #21945

Change-Id: Ie991f9bc1afda2b49ac5d734eb41c37d3a37e554
Reviewed-on: https://go-review.googlesource.com/72371
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>