bootstrap-tools-cross: Rename bootstrap file jobs to be more descriptive

Rename files in `bootstrap-files/` to match jobs in `make-bootstrap-tools-cross`.

Should make automating the bootstrap files easier.

```nix
$ nix repl --file ./pkgs/top-level/release-cross.nix
nix-repl> bootstrapTools
{ aarch64-unknown-linux-gnu = { ... }; aarch64-unknown-linux-musl = { ... }; armv5tel-unknown-linux-gnueabi = { ... }; armv6l-unknown-linux-gnueabihf = { ... }; armv6l-unknown-linux-musleabihf = { ... }; armv7l-unknown-linux-gnueabihf = { ... }; mips64el-unknown-linux-gnuabi64 = {... }; mips64el-unknown-linux-gnuabin32 = { ... }; mipsel-unknown-linux-gnu = { ... }; powerpc64le-unknown-linux-gnu = { ... }; riscv64-unknown-linux-gnu = { ... }; x86_64-unknown-linux-musl = { ... }; }
```

Additionally if non-linux bootstrap files are added to
`make-bootstrap-tools-cross` then there won't be any renaming needed.

pkgs/stdenv/linux/bootstrap-files/x86_64.nix pkgs/stdenv/linux/bootstrap-files/x86_64-unknown-linux-gnu.nix

···
 # Use busybox for i686-linux since it works on x86_64-linux as well.
-(import ./i686.nix) //
+(import ./i686-unknown-linux-gnu.nix) //
 
 {
   bootstrapTools = import <nix/fetchurl.nix> {

pkgs/stdenv/linux/default.nix

···
 , bootstrapFiles ?
   let table = {
     glibc = {
-      i686-linux = import ./bootstrap-files/i686.nix;
-      x86_64-linux = import ./bootstrap-files/x86_64.nix;
-      armv5tel-linux = import ./bootstrap-files/armv5tel.nix;
-      armv6l-linux = import ./bootstrap-files/armv6l.nix;
-      armv7l-linux = import ./bootstrap-files/armv7l.nix;
-      aarch64-linux = import ./bootstrap-files/aarch64.nix;
-      mipsel-linux = import ./bootstrap-files/mipsel.nix;
+      i686-linux = import ./bootstrap-files/i686-unknown-linux-gnu.nix;
+      x86_64-linux = import ./bootstrap-files/x86_64-unknown-linux-gnu.nix;
+      armv5tel-linux = import ./bootstrap-files/armv5tel-unknown-linux-gnueabi.nix;
+      armv6l-linux = import ./bootstrap-files/armv6l-unknown-linux-gnueabihf.nix;
+      armv7l-linux = import ./bootstrap-files/armv7l-unknown-linux-gnueabihf.nix;
+      aarch64-linux = import ./bootstrap-files/aarch64-unknown-linux-gnu.nix;
+      mipsel-linux = import ./bootstrap-files/mipsel-unknown-linux-gnu.nix;
       mips64el-linux = import
        (if localSystem.isMips64n32
-        then ./bootstrap-files/mips64el-n32.nix
-        else ./bootstrap-files/mips64el.nix);
-      powerpc64le-linux = import ./bootstrap-files/powerpc64le.nix;
-      riscv64-linux = import ./bootstrap-files/riscv64.nix;
+        then ./bootstrap-files/mips64el-unknown-linux-gnuabin32.nix.nix
+        else ./bootstrap-files/mips64el-unknown-linux-gnuabi64.nix);
+      powerpc64le-linux = import ./bootstrap-files/powerpc64le-unknown-linux-gnu.nix;
+      riscv64-linux = import ./bootstrap-files/riscv64-unknown-linux-gnu.nix;
     };
     musl = {
-      aarch64-linux = import ./bootstrap-files/aarch64-musl.nix;
-      armv6l-linux  = import ./bootstrap-files/armv6l-musl.nix;
-      x86_64-linux  = import ./bootstrap-files/x86_64-musl.nix;
+      aarch64-linux = import ./bootstrap-files/aarch64-unknown-linux-musl.nix;
+      armv6l-linux  = import ./bootstrap-files/armv6l-unknown-linux-musleabihf.nix;
+      x86_64-linux  = import ./bootstrap-files/x86_64-unknown-linux-musl.nix;
     };
   };
 

pkgs/stdenv/linux/make-bootstrap-tools-cross.nix

···
   # NOTE: Only add platforms for which there are files in `./bootstrap-files`.
   # Sort following the sorting in `./default.nix` `bootstrapFiles` argument.
 
-  armv5tel   = sheevaplug;
-  armv6l     = raspberryPi;
-  armv7l     = armv7l-hf-multiplatform;
-  aarch64    = aarch64-multiplatform;
-  mipsel-linux-gnu         = mipsel-linux-gnu;
-  mips64el-linux-gnuabin32 = mips64el-linux-gnuabin32;
-  mips64el-linux-gnuabi64  = mips64el-linux-gnuabi64;
-  powerpc64le = powernv;
-  riscv64 = riscv64;
+  armv5tel-unknown-linux-gnueabi = sheevaplug;
+  armv6l-unknown-linux-gnueabihf = raspberryPi;
+  armv7l-unknown-linux-gnueabihf = armv7l-hf-multiplatform;
+  aarch64-unknown-linux-gnu = aarch64-multiplatform;
+  mipsel-unknown-linux-gnu = mipsel-linux-gnu;
+  mips64el-unknown-linux-gnuabin32 = mips64el-linux-gnuabin32;
+  mips64el-unknown-linux-gnuabi64 = mips64el-linux-gnuabi64;
+  powerpc64le-unknown-linux-gnu = powernv;
+  riscv64-unknown-linux-gnu = riscv64;
 
   # musl
-  aarch64-musl = aarch64-multiplatform-musl;
-  armv6l-musl  = muslpi;
-  x86_64-musl  = musl64;
+  aarch64-unknown-linux-musl = aarch64-multiplatform-musl;
+  armv6l-unknown-linux-musleabihf = muslpi;
+  x86_64-unknown-linux-musl = musl64;
 })