lib/systems/platforms.nix: fix broken mips32 detection
lib/systems/platforms.nix: fix broken mips32 detection

Prior to this commit, nixpkgs would assume that every little-endian
mips32 system was a "fuloong2f_n32".

Not only are there plenty of mips32 chips other than the fuloong, but
the fuloong is actually a mips64 chip!  Note that the "n32" ABI is
(confusingly) an ABI for 64-bit mips chips (like the "x32" ABI for
amd64 chips -- both are ABIs which use 32-bit pointers on an
otherwise-64-bit system).

This error causes far-ranging problems.  One of them was particularly
difficult to track down: it caused GCC to select 128-bit `long double`
types, which is invalid for the mips32 ABI.  This isn't noticed until
you try to build musl-libc, which is careful to check for these things.

Prior to this commit,

  nix-build . -A pkgsCross.mipsel-linux-gnu.pkgsStatic.hello

would fail.  With this commit and #170736, it succeeds.

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