Build system: make clean in utils/ghc-pkg should not delete inplace/lib/bin
Build system: make clean in utils/ghc-pkg should not delete inplace/lib/bin

Make sure $1_$2_PROG always gets assigned a value, even when cleaning.

The problem with not setting the variable becomes apparent when looking
at the following two lines of code:

```
  $1_$2_INPLACE = $$(INPLACE_LIB)/bin/$$($1_$2_PROG)
  $(call clean-target,$1,$2_inplace,$$($1_$2_INPLACE))
```

So running `make clean` in for example `utils/ghc-pkg` deletes
`inplace/lib/bin/` instead of `inplace/lib/bin/ghc-pkg`.

The offending code was introduced in commit
2b85372ca18115bb1d6363256fcea6f54e415bed.

There is one small implication. When cleaning before configure, the
variable $1_$2_PROG will now be assigned a slightly wrong value, because
exeext$3 isn't known yet. But I think that's ok, as no files have been
build yet, so it will just try to delete a slighly different nonexistent
file.

[skip ci]

Differential Revision: https://phabricator.haskell.org/D916

mfine/ghc,GaloisInc/halvm-ghc,acowley/ghc,vTurbine/ghc,ml9951/ghc,da-x/ghc,nkaretnikov/ghc,siddhanathan/ghc,urbanslug/ghc,wxwxwwxxx/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,vikraman/ghc,nushio3/ghc,nushio3/ghc,anton-dessiatov/ghc,AlexanderPankiv/ghc,tjakway/ghcjvm,acowley/ghc,ghc-android/ghc,ezyang/ghc,vTurbine/ghc,sgillespie/ghc,snoyberg/ghc,ezyang/ghc,olsner/ghc,nushio3/ghc,vikraman/ghc,urbanslug/ghc,ml9951/ghc,mettekou/ghc,nkaretnikov/ghc,olsner/ghc,mcschroeder/ghc,mfine/ghc,sdiehl/ghc,AlexanderPankiv/ghc,ghc-android/ghc,snoyberg/ghc,siddhanathan/ghc,sdiehl/ghc,TomMD/ghc,ghc-android/ghc,elieux/ghc,snoyberg/ghc,da-x/ghc,mfine/ghc,oldmanmike/ghc,mfine/ghc,oldmanmike/ghc,siddhanathan/ghc,gridaphobe/ghc,acowley/ghc,ghc-android/ghc,mcschroeder/ghc,wxwxwwxxx/ghc,urbanslug/ghc,TomMD/ghc,olsner/ghc,oldmanmike/ghc,shlevy/ghc,ml9951/ghc,nkaretnikov/ghc,sgillespie/ghc,mettekou/ghc,da-x/ghc,sdiehl/ghc,da-x/ghc,da-x/ghc,ml9951/ghc,gridaphobe/ghc,nushio3/ghc,mcschroeder/ghc,tjakway/ghcjvm,nkaretnikov/ghc,wxwxwwxxx/ghc,sgillespie/ghc,acowley/ghc,shlevy/ghc,GaloisInc/halvm-ghc,elieux/ghc,urbanslug/ghc,snoyberg/ghc,vTurbine/ghc,TomMD/ghc,nkaretnikov/ghc,anton-dessiatov/ghc,mcschroeder/ghc,wxwxwwxxx/ghc,olsner/ghc,mcschroeder/ghc,ezyang/ghc,urbanslug/ghc,ml9951/ghc,gridaphobe/ghc,TomMD/ghc,vTurbine/ghc,vikraman/ghc,acowley/ghc,elieux/ghc,GaloisInc/halvm-ghc,wxwxwwxxx/ghc,ghc-android/ghc,TomMD/ghc,elieux/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,olsner/ghc,vikraman/ghc,ezyang/ghc,gridaphobe/ghc,anton-dessiatov/ghc,snoyberg/ghc,mettekou/ghc,siddhanathan/ghc,sdiehl/ghc,elieux/ghc,anton-dessiatov/ghc,ml9951/ghc,urbanslug/ghc,vTurbine/ghc,mettekou/ghc,AlexanderPankiv/ghc,urbanslug/ghc,AlexanderPankiv/ghc,shlevy/ghc,vikraman/ghc,oldmanmike/ghc,anton-dessiatov/ghc,siddhanathan/ghc,mfine/ghc,elieux/ghc,gridaphobe/ghc,elieux/ghc,wxwxwwxxx/ghc,sgillespie/ghc,shlevy/ghc,anton-dessiatov/ghc,tjakway/ghcjvm,vikraman/ghc,nushio3/ghc,ml9951/ghc,shlevy/ghc,mcschroeder/ghc,oldmanmike/ghc,TomMD/ghc,AlexanderPankiv/ghc,tjakway/ghcjvm,GaloisInc/halvm-ghc,mettekou/ghc,oldmanmike/ghc,vTurbine/ghc,anton-dessiatov/ghc,sgillespie/ghc,siddhanathan/ghc,ghc-android/ghc,tjakway/ghcjvm,vTurbine/ghc,mfine/ghc,ghc-android/ghc,olsner/ghc,snoyberg/ghc,AlexanderPankiv/ghc,shlevy/ghc,nushio3/ghc,gridaphobe/ghc,gridaphobe/ghc,acowley/ghc,tjakway/ghcjvm,ezyang/ghc,shlevy/ghc,sgillespie/ghc,vikraman/ghc,sgillespie/ghc,ml9951/ghc,GaloisInc/halvm-ghc,sdiehl/ghc,mfine/ghc,da-x/ghc,nkaretnikov/ghc,AlexanderPankiv/ghc,nushio3/ghc,ezyang/ghc,GaloisInc/halvm-ghc,siddhanathan/ghc,snoyberg/ghc,olsner/ghc,wxwxwwxxx/ghc,acowley/ghc,mettekou/ghc,TomMD/ghc,sdiehl/ghc,nkaretnikov/ghc,da-x/ghc,oldmanmike/ghc,mcschroeder/ghc