gcc: fix Hydra builds for bootstrapTools.mips64el-linux-gnuabin32

[ghost]

Description of changes

The Hydra builds for bootstrapTools.mips64el-linux-gnuabin32.build are currently failing due to make-bootstrap-files.nix expecting to find libraries in $PREFIX/lib, whereas on mips gcc uses the slightly awkward IRIX convention:

$PREFIX/lib   = mips32
$PREFIX/lib32 = mips64n32
$PREFIX/lib64 = mips64n64

We already work around a similar problem on x86_64 by creating symlinks from lib64 to lib. This PR disables that workaround if isMips64n32 and enables an analogous workaround to link lib32 to lib.

Things done

• Built on platform(s)
  • powerpc64le-linux (cross to mips64n32)
  • mips64n32-linux (bootstrap from newly-built bootstrap-files, up to hello)
• Tested compilation of many packages that depend on this change
• Tested basic functionality of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

[ghost]

Using bootstrap-files built from this PR:

nix build -f . -L --arg localSystem '(import ./lib).systems.examples.mips64el-linux-gnuabin32' hello
...
hello> make[3]: Leaving directory '/build/hello-2.12'
hello> make[2]: Leaving directory '/build/hello-2.12'
hello> make[1]: Leaving directory '/build/hello-2.12'
hello> post-installation fixup
hello> shrinking RPATHs of ELF executables and libraries in /nix/store/5d04x57amfyc1sb42g21jrjxfhx2pd4n-hello-2.12
hello> shrinking /nix/store/5d04x57amfyc1sb42g21jrjxfhx2pd4n-hello-2.12/bin/hello
hello> gzipping man pages under /nix/store/5d04x57amfyc1sb42g21jrjxfhx2pd4n-hello-2.12/share/man/
hello> strip is /nix/store/29jinn47ar9s1brdyi8qfyb8n74g63k8-gcc-wrapper-10.4.0/bin/strip
hello> stripping (with command strip and flags -S) in /nix/store/5d04x57amfyc1sb42g21jrjxfhx2pd4n-hello-2.12/bin
hello> patching script interpreter paths in /nix/store/5d04x57amfyc1sb42g21jrjxfhx2pd4n-hello-2.12
hello> checking for references to /build/ in /nix/store/5d04x57amfyc1sb42g21jrjxfhx2pd4n-hello-2.12...

nix@oak:/nix/nixpkgs$ result/bin/hello
Hello, world!

[ghost]

Note that the nix binary in the example above needs to be built with NixOS/nix#6456 (or else use --disable-syscall-filters).

[ghost]

This Hydra build is the first one to include this PR, so it ought to produce a working bootstrap-files.

[vcunat]

Hmm, yes: https://hydra.nixos.org/eval/1777726?filter=mips
Some failing jobs remain, but those steps probably shouldn't be attempted and "fix" would be by removing them (or something similar).

[ghost]

This Hydra build is the first one to include this PR, so it ought to produce a working bootstrap-files.

Nice: #188334

Hmm, yes: https://hydra.nixos.org/eval/1777726?filter=mips
Some failing jobs remain, but those steps probably shouldn't be attempted and "fix" would be by removing them (or something similar).

When I click the link above, I see the failures, all for attrnames of the form bootstrapTools.${PLATFORM}.bootstrapTools.

Hydra should never attempt to build bootstrapTools.${PLATFORM}.bootstrapTools unless with stdenv; buildPlatform.canExecute hostPlatform.

Indeed, there are a ton of Hydra failures in this category; I count 17 of them:

https://hydra.nixos.org/eval/1777726?filter=.bootstrapTools&compare=1777572&full=#tabs-still-fail

I'm preparing a PR to fix this.

[ghost]

Indeed, there are a ton of Hydra failures in this category; I count 17 of them:

The fix is #188339.