Revert "Make the option of building using the host clang the default"

This reverts commit 0b1e858.

No one remembers what problems this caused for the ASan bot when we
tried building the runtime with the just-built clang, so I'm reenabling
it now and we'll see what happens.

Author: bob-wilson

utils/build-script-impl

@@ -81,7 +81,7 @@ KNOWN_SETTINGS=(
     cmake                       ""               "path to the cmake binary"
     distcc                      ""               "use distcc in pump mode"
     distcc-pump                 ""               "the path to distcc pump executable. This argument is required if distcc is set."
-    build-runtime-with-host-compiler   "1"       "use the host c++ compiler to build everything"
+    build-runtime-with-host-compiler   ""        "use the host c++ compiler to build everything"
     cmake-generator             "Unix Makefiles" "kind of build system to generate; see output of 'cmake --help' for choices"
     verbose-build               ""               "print the commands executed during the build"
     install-prefix              ""               "installation prefix"