misc setup-hooks: Use env vars to refer to binutils programs

This is more robust for cross-compilation

pkgs/build-support/setup-hooks/separate-debug-info.sh

···
         if ! isELF "$i"; then continue; fi
 
         # Extract the Build ID. FIXME: there's probably a cleaner way.
-        local id="$(readelf -n "$i" | sed 's/.*Build ID: \([0-9a-f]*\).*/\1/; t; d')"
+        local id="$($READELF -n "$i" | sed 's/.*Build ID: \([0-9a-f]*\).*/\1/; t; d')"
         if [ "${#id}" != 40 ]; then
             echo "could not find build ID of $i, skipping" >&2
             continue
···
         # Extract the debug info.
         header "separating debug info from $i (build ID $id)"
         mkdir -p "$dst/${id:0:2}"
-        objcopy --only-keep-debug "$i" "$dst/${id:0:2}/${id:2}.debug"
-        strip --strip-debug "$i"
+        $OBJCOPY --only-keep-debug "$i" "$dst/${id:0:2}/${id:2}.debug"
+        $STRIP --strip-debug "$i"
 
         # Also a create a symlink <original-name>.debug.
         ln -sfn ".build-id/${id:0:2}/${id:2}.debug" "$dst/../$(basename "$i")"

pkgs/build-support/setup-hooks/strip.sh

···
 
     if [ -n "${dirs}" ]; then
         header "stripping (with flags $stripFlags) in$dirs"
-        find $dirs -type f -print0 | xargs -0 ${xargsFlags:--r} strip $commonStripFlags $stripFlags 2>/dev/null || true
+        find $dirs -type f -print0 | xargs -0 ${xargsFlags:--r} $STRIP $commonStripFlags $stripFlags 2>/dev/null || true
         stopNest
     fi
 }

pkgs/build-support/setup-hooks/win-dll-link.sh

···
     linkCount=0
     # Iterate over any DLL that we depend on.
     local dll
-    for dll in $(objdump -p *.{exe,dll} | sed -n 's/.*DLL Name: \(.*\)/\1/p' | sort -u); do
+    for dll in $($OBJDUMP -p *.{exe,dll} | sed -n 's/.*DLL Name: \(.*\)/\1/p' | sort -u); do
         if [ -e "./$dll" ]; then continue; fi
         # Locate the DLL - it should be an *executable* file on $DLLPATH.
         local dllPath="$(PATH="$DLLPATH" type -P "$dll")"