newlib: switch to multilib.am

newlib: switch to multilib.am

We use the common config-ml.in for configure, so switch the makefile
over to the common multilib.am.  It's almost exactly the same code,
but there are two differences:
* Common code hooks install-exec-local for install-multi, but newlib
  doesn't currently install any executables, so that doesn't fire.
  Newlib already has install-data-local that inlined install-multi,
  so switch that to the common install-multi.
* Common code doesn't provide a check-multi at all.  Keep ours for
  now.  Some day common code might get it.  Or not.  Who knows.