Verify that the linker supports the DWARF v4 spec

This change goes one step further in testing for coverage support by ensuring
that the linker is LLD and supports DWARF v4.

This is a proper fix for the arm/armv6 case not supporting --coverage with
bfd.

As a side effect, this change intentionally blacklists --coverage support
with binutils, i.e., the raw gcc toolchain. I will need to figure out what
versions of binutils support DWARF v4, etc, then add support for them after
verifying that they function with --coverage, most likely using an
xtoolchain.