Let's remove AA_APPLE_UNIVERSAL_BUILD from gl_MULTIARCH

[Paul Eggert]

m4/multiarch.m4's gl_MULTIARCH macro defines
AA_APPLE_UNIVERSAL_BUILD, but as far as I can
tell nobody uses that symbol.  I propose that we remove
it, as unnecessary.  (I'm trying to put Emacs src/config.h
on a diet and this would remove one more unnecesary symbol
from Emacs.)

Autoconf's AC_C_BIGENDIAN defines AC_APPLE_UNIVERSAL_BUILD
in order to get WORDS_BIGENDIAN right, and I expect this
was copied to gnulib and renamed at some point, but that the
need for the symbol in gnulib (if there ever was one) has
gone away.