uclibc-ng: fix cross-compile to mips (for ben-nanonote)
uclibc-ng: fix cross-compile to mips (for ben-nanonote)

Must specify the the TARGET_ARCH (which confusingly is actually
for the host platform, in Nix/autotools terminology) at build and
`make install` time.

Also, auto-disable UCLIBC_HAS_FPU when hostPlatform.gcc.float == "soft".

Clarify platforms = linux and broken = arm to better reflect
the current support situation.

Fixes pkgsCross.ben-nanonote.stdenv

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs