Log Message:
binutils: Improve gas/gld handling and cleanup.

Add a new "disable-ld" option that disables ld.  This is required on Darwin
where it is not currently built, but is also required for some SunOS setups
where an external GCC is used and we do not want a libgcc dependency on it.

There are now two ld-related PLIST variables, "gld" for the general \ 
handling
of whether GNU ld is built or not, and "ld" for the symlinks we create.

In addition Darwin also needs --disable-libctf to avoid a build issue, and an
additional PLIST variable to handle GNU as which is also not built on Darwin.

While here clean up a bunch of variable handling and NetBSD 5/6 workarounds,
and simplify the setup of SYMLINK_FILES.

Fixes build on Darwin, and SunOS when using an external GCC.  Ok fcambus@