more robust reference to ghc exe (with multi-platform support);

--- a/lib/Tools/ghc_setup	Wed Mar 20 21:54:08 2019 +0100
+++ b/lib/Tools/ghc_setup	Wed Mar 20 21:57:02 2019 +0100
@@ -8,4 +8,4 @@
 
 isabelle_stack setup
 isabelle_stack ghc -- --version
-echo -n "$(standard_path "$(isabelle_stack path --programs)")" > "$ISABELLE_STACK_ROOT/ISABELLE_GHC_PROGRAMS"
+echo -n "$(standard_path "$(isabelle_stack path --compiler-exe)")" > "$ISABELLE_STACK_ROOT/ISABELLE_GHC_EXE-$ISABELLE_PLATFORM_FAMILY"

--- a/lib/scripts/getsettings	Wed Mar 20 21:54:08 2019 +0100
+++ b/lib/scripts/getsettings	Wed Mar 20 21:57:02 2019 +0100
@@ -105,8 +105,8 @@
 fi
 
 #enforce ISABELLE_GHC
-if [ -d "$ISABELLE_STACK_ROOT" -a -f "$ISABELLE_STACK_ROOT/ISABELLE_GHC_PROGRAMS" ]; then
-  if [ -f "$(cat "$ISABELLE_STACK_ROOT/ISABELLE_GHC_PROGRAMS")/$ISABELLE_GHC_VERSION/bin/ghc" ]; then
+if [ -f "$ISABELLE_STACK_ROOT/ISABELLE_GHC_EXE-$ISABELLE_PLATFORM_FAMILY" ]; then
+  if [ -f "$(cat "$ISABELLE_STACK_ROOT/ISABELLE_GHC_EXE-$ISABELLE_PLATFORM_FAMILY")" ]; then
     ISABELLE_GHC="$ISABELLE_HOME/lib/scripts/ghc"
     ISABELLE_GHC_STACK=true
   fi