In b592bd98ff25730bbe3c13d6f62a427df8c78e28 we started using
-dead_strip_dylib on macOS when lining dynamic libraries and binaries.
The underlying reason being the Load Command Size Limit in macOS
Sierra (10.14) and later.

GHC will produce @rpath/libHS... dependency entries together with a
corresponding RPATH entry pointing to the location of the libHS...
library. Thus for every library we produce two Load Commands.  One to
specify the dependent library, and one with the path where to find it.
This makes relocating libraries and binaries easier, as we just need to
update the RPATH entry with the install_name_tool. The dynamic linker
will then subsitute each @rpath with the RPATH entries it finds in the
libraries load commands or the environement, when looking up @rpath
relative libraries.

-dead_strip_dylibs intructs the linker to drop unused libraries. This in
turn help us reduce the number of referenced libraries, and subsequently
the size of the load commands.  This however does not remove the RPATH
entries.  Subsequently we can end up (in extreme cases) with only a
single @rpath/libHS... entry, but 100s or more RPATH entries in the Load
Commands.

This patch rectifies this (slighly unorthodox) by passing *no* -rpath
arguments to the linker at link time, but -headerpad 8000.  The
headerpad argument is in hexadecimal and the maxium 32k of the load
command size.  This tells the linker to pad the load command section
enough for us to inject the RPATHs later.  We then proceed to link the
library or binary with -dead_strip_dylibs, and *after* the linking
inspect the library to find the left over (non-dead-stripped)
dependencies (using otool).  We find the corresponding RPATHs for each
@rpath relative dependency, and inject them into the library or binary
using the install_name_tool.  Thus achieving a deadstripped dylib (and
rpaths) build product.

We can not do this in GHC, without starting to reimplement a dynamic
linker as we do not know which symbols and subsequently libraries are
necessary.

Commissioned-by: Mercury Technologies, Inc. (mercury.com)

The fix to #17962 ended up regressing on Windows as it failed to
replicate the logic responsible for overriding the toolchain paths on
Windows. This resulted in a hard-coded path to a directory that likely
doesn't exist on the user's system (#18550).

Previously to merge a set of object files we would invoke the linker as
usual, adding -r to the command-line. However, this can result in
non-sensical command-lines which causes lld to balk (#17962).

To avoid this we introduce a new tool setting into GHC, -pgmlm, which is
the linker which we use to merge object files.

* Represent backends with a `Backend` datatype in GHC.Driver.Backend

* Don't detect the default backend to use for the target platform at
  compile time in Hadrian/make but at runtime. It makes "Settings"
  simpler and it is a step toward making GHC multi-target.

* The latter change also fixes hadrian which has not been updated to
  take into account that the NCG now supports AIX and PPC64 (cf
  df26b95559fd467abc0a3a4151127c95cb5011b9 and
  d3c1dda60d0ec07fc7f593bfd83ec9457dfa7984)

* Also we don't treat iOS specifically anymore (cf
  cb4878ffd18a3c70f98bdbb413cd3c4d1f054e1f)

This fixes the error message given by configure when the user
attempts to configure without first download the win32 tarballs.

Previously we would simply assume that makeindex was available.
Now we correctly detect it in `configure` and respect this conclusion in
hadrian and make.

* replace integer-* package selection with ghc-bignum backend selection