The availability of native codegen (which allows us to disable the LLVM
backend by default) hinges on the target platform of the compiler (that
is GHC), not on the platform it runs on (the host platform).

(cherry picked from commit cfdc073)

Based on https://gitlab.haskell.org/ghc/ghc/-/wikis/platforms, although
it sadly doesn't list when the backends were introduced.

* PowerPC, x86 (and x86_64) and Sparc have been supported for longer.

* aarch64-darwin is new in 9.2.1 and backported to 8.10.5, 8.10.6 and
  8.10.7 (check is dumb here since we'll grep for 8.10.7 anyways when
  upgrading)

Fixes ghcWithPackages failing to evaluate on aarch64-darwin because of
missing support for the platform in LLVM 9's compiler-rt.

(cherry picked from commit 571f3e5)

This check was wrong and caused by a bit of confusion on my part.
GHC >= 8.10.5 && < 9 supports aarch64-darwin via LLVM and GHC >= 9.2.1
introduces the NCG backend for aarch64-darwin.

(cherry picked from commit 2f98c18)

useLLVM is what we are using in the GHC derivations already -- for
better or for worse -- so we should rename the argument here for
consistency which we are free to do as this is purely internal at the
moment (with overriding being impossible).

(cherry picked from commit c32095b)

See https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/compiler/backends/llvm/installing#llvm-support

(cherry picked from commit e191321)

This means we only have to update the llvmPackages attribute in one
place now and should prevent situations like with 8.6.5 where different
versions would be used in the package set compared to the compiler
build.

Drop comments in the configuration-ghc-X.Y.x.nix files as well, since
LLVM version isn't tied to the compiler minor version at
all (e. g. 8.10.2 and 8.10.7 have different support ranges).

(cherry picked from commit d7ff806)

GHC 9.0.1 only supports LLVM 9 and spews a lot of warnings about LLVM 10
when using the LLVM backend atm.

See also: https://www.haskell.org/ghc/download_ghc_9_0_1.html

(cherry picked from commit 5a568ea)

Source:

* (8.6.5: https://www.haskell.org/ghc/download_ghc_8_6_5.html)
* (8.8.4: https://www.haskell.org/ghc/download_ghc_8_8_4.html)
* (8.10.2: https://www.haskell.org/ghc/download_ghc_8_10_2.html)
* 8.10.7: https://www.haskell.org/ghc/download_ghc_8_10_7.html
* (9.0.1: https://www.haskell.org/ghc/download_ghc_9_0_1.html)
* 9.2.1: https://www.haskell.org/ghc/download_ghc_9_2_1.html
* HEAD: https://gitlab.haskell.org/ghc/ghc/-/blob/3ab3631f41efe9a1ae32a5b1f709152228edb09a/configure.ac#L674

(cherry picked from commit 9e1f438)

GHC 8.6.5 will always segfault on aarch64-linux and at this point
it's not realistic we'll ever fix this.

(cherry picked from commit b9f1582)

Since LLVM itself doesn't depend on target at all, this doesn't change
anything *in effect* (i. e. rebuild count should be zero), but it is
more clear about the intention and what LLVM is used for here (i. e. in
depsBuildTarget).

(cherry picked from commit 156d8d6)

Since we inherit the platform list from the bootstrap GHC, we get
differing lists depending on which platform we evaluate the platform
list on (depending on whether 8.10.2 or 8.6.5 is used). This leads to
Hydra thinking aarch64-linux is not supported as it evaluates on
x86_64-linux usually.

(cherry picked from commit 55b8d8c)

(cherry picked from commit b2c2215)

This will prevent freak accidents where the wrong tools are used because
they are in PATH by chance.

(cherry picked from commit 035f20b)

(cherry picked from commit a7c5645)

aarch64-darwin NCG was added in 9.2.1 which makes it unnecessary to
include LLVM in the wrapper.

(cherry picked from commit ef081bf)