add m68k port

three ABIs are supported: the default with 68881 80-bit fpu format and
results returned in floating point registers, softfloat-only with the
same format, and coldfire fpu with IEEE single/double only. only the
first is tested at all, and only under qemu which has fpu emulation
bugs.

basic functionality smoke tests have been performed for the most
common arch-specific breakage via libc-test and qemu user-level
emulation. some sysvipc failures remain, but are shared with other big
endian archs and will be fixed separately.
diff --git a/configure b/configure
index 09a0c43..f940af9 100755
--- a/configure
+++ b/configure
@@ -320,6 +320,7 @@
 x86_64-x32*|x32*|x86_64*x32) ARCH=x32 ;;
 x86_64-nt64*) ARCH=nt64 ;;
 x86_64*) ARCH=x86_64 ;;
+m68k*) ARCH=m68k ;;
 mips64*|mipsisa64*) ARCH=mips64 ;;
 mips*) ARCH=mips ;;
 microblaze*) ARCH=microblaze ;;
@@ -641,6 +642,13 @@
 trycppif __AARCH64EB__ "$t" && SUBARCH=${SUBARCH}_be
 fi
 
+if test "$ARCH" = "m68k" ; then
+if trycppif "__HAVE_68881__" ; then : ;
+elif trycppif "__mcffpu__" ; then SUBARCH="-fp64"
+else SUBARCH="-sf"
+fi
+fi
+
 if test "$ARCH" = "mips" ; then
 trycppif "__mips_isa_rev >= 6" "$t" && SUBARCH=${SUBARCH}r6
 trycppif "_MIPSEL || __MIPSEL || __MIPSEL__" "$t" && SUBARCH=${SUBARCH}el