utf8.h: Add new #define for extended length UTF-8

The previous commit added a convenient place to create a symbol to
indicate that the UTF-8 on this platform includes Perl's nearly-double
length extension.  The platforms this isn't needed on are 32-bit ASCII
ones.  This symbol allows removing one place where EBCDIC need
be considered, and future commits will use it as well.

utf8.c

@@ -894,7 +894,7 @@ S_does_utf8_overflow(const U8 * const s,
 #  endif
 #endif
 
-#if ! defined(UV_IS_QUAD) && ! defined(EBCDIC)
+#ifndef HAS_EXTRA_LONG_UTF8
 
     /* On 32 bit ASCII machines, many overlongs that start with FF don't
      * overflow */

utf8.h

@@ -448,6 +448,7 @@ regen/charset_translations.pl. */
  * platforms, that doesn't happen until 6*6 bits, so on those platforms, this
  * will always be false */
 #if UVSIZE * CHARBITS > (6 * UTF_CONTINUATION_BYTE_INFO_BITS)
+#  define HAS_EXTRA_LONG_UTF8
 #  define OFFUNISKIP_helper_(uv)                                            \
      UNLIKELY(uv > nBIT_UMAX(6 * UTF_CONTINUATION_BYTE_INFO_BITS))          \
       ? UTF8_MAXBYTES :