binutils: Don't pass --host etc. when not cross-compiling

Or linuxHeaders build fails on ARM (non-cross) since e826a6a2:

````
  HOSTCC  scripts/basic/fixdep
/nix/store/9glws7v2j28blv1n7azqr3b43qadjd1j-binutils-2.28.1/bin/ld: unrecognised emulation mode: armelf_linux_eabi
Supported emulations: armelf_linux armelf armelfb armelfb_linux
collect2: error: ld returned 1 exit status
````