Switch to using gold linker by default

Switch to using gold linker by default

DragonFly has always used the "GNU ld" linker to build itself.  Now that
the ELF boot loader has been fixed to handle zero-offset headers (seen
with zero-length ELF notes), the newer gold linker can handle being the
default.  That's what this commit does.

People can continue with the classic linker by putting the following in
/etc/make.conf:  WORLD_LDVER=ld.bfd
... and setting "LDVER=ld.bfd" in their environment.

The gold linker does not use the bfd library and thus is limited to the
ELF format (fine for DF).  It's code is much cleaner (c++), so it's easy
to understand and modify, and it reportedly links complex c++ object
files significantly faster than the BFD-based linker.  Both linkers are
written by the same person, Ian Lance Taylor.