libgloss: restore multilib settings in subdir makefiles

libgloss: restore multilib settings in subdir makefiles

Commit 754f8def0dfeeb43afa5a96ad1971fd0ef02c419 ("libgloss: merge stub
arch configure scripts up a level") had an unintended side-effect: the
MULTI* variables in the Makefiles no longer get rewritten at configure
time in the subdirs.  Only the top-level Makefile still is.  This is
because the configure integration of multilib (both the way libgloss
did it manually and the way AM_ENABLE_MULTILIB does it) only rewrites
"Makefile".

We could try propagating the MULTI* variables from libgloss/Makefile
down via FLAGS_TO_PASS, but this runs into a limitation: the multilib
logic uses this variable to switch from libgloss/ to each multilib
libgloss/, and libgloss uses this variable to enter subdirectories.
The latter we want, but the former ends up overridding the Makefile
environment.  We could side-step that with some GNU Make directives,
but it feels like we're getting a bit too deep down the rabbit hole.

Instead, let's call config-ml.in ourselves for each subdir Makefile
that the top-level configure generates.  This restores the previous
behavior and should be less risky in general.

This logic should be unnecessary when/if we switch libgloss over to
a non-recursive Automake build (since all build+install settings are
in the single libgloss/Makefile), but it'll be a while before we can
land that rework, so let's fix this up now.