pkgsStatic: fix musleabi* adapter
pkgsStatic: fix musleabi* adapter

Fixes pkgsCross.muslpi.nix (depends on
pkgsCross.muslpi.busybox-sandbox-shell, which depends on
pkgsCross.muslpi.pkgsStatic.stdenv)

Currently, the ABI adapter in pkgsStatic does not
recognize musleabi/musleabihf and falls back to "musl".

```
> nix eval -f . pkgsCross.muslpi.stdenv.hostPlatform.config
"armv6l-unknown-linux-musleabihf"
> nix eval -f . pkgsCross.muslpi.pkgsStatic.stdenv.hostPlatform.config
"armv6l-unknown-linux-musl"
```

This results in an invalid config passed to GCC configuration, which
fails with `Configuration armv6l-unknown-linux-musl not supported`.

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