add arch/generic include fallback to build rules

this sets the stage for the first phase of the bits deduplication.
bits headers which are identical for "most" archs will be moved to
arch/generic/bits.
diff --git a/configure b/configure
index 41ef9df..a296f8a 100755
--- a/configure
+++ b/configure
@@ -651,8 +651,9 @@
 echo '#if LDBL_MANT_DIG == 53' >> "$tmpc"
 echo 'typedef char ldcheck[9-(int)sizeof(long double)];' >> "$tmpc"
 echo '#endif' >> "$tmpc"
-if $CC $CFLAGS_C99FSE -I$srcdir/arch/$ARCH -I$srcdir/include $CPPFLAGS $CFLAGS \
-  -c -o /dev/null "$tmpc" >/dev/null 2>&1 ; then
+if $CC $CFLAGS_C99FSE \
+  -I$srcdir/arch/$ARCH -I$srcdir/arch/generic -I$srcdir/include \
+  $CPPFLAGS $CFLAGS -c -o /dev/null "$tmpc" >/dev/null 2>&1 ; then
 printf "yes\n"
 else
 printf "no\n"