lib: Clean up float/fpu options

ARM ABIs now have a float field. This is used as a fallback to lessen
our use of `platform.gcc.float`. I didn't know what the MIPs convention
is so I kept using `platform.gcc.float` in that case.

pkgs/development/compilers/gcc/common/platform-flags.nix

@@ -2,11 +2,12 @@
 
 let
   p = targetPlatform.platform.gcc or {};
+  float = p.float or (targetPlatform.parsed.abi.float or null);
 in lib.concatLists [
   (lib.optional (p ? arch) "--with-arch=${p.arch}")
   (lib.optional (p ? cpu) "--with-cpu=${p.cpu}")
   (lib.optional (p ? abi) "--with-abi=${p.abi}")
   (lib.optional (p ? fpu) "--with-fpu=${p.fpu}")
-  (lib.optional (p ? float) "--with-float=${p.float}")
+  (lib.optional (float != null) "--with-float=${float}")
   (lib.optional (p ? mode) "--with-mode=${p.mode}")
 ]