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

The previous commit adds 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 a couple of places where EBCDCIC need
be considered.

inline.h

@@ -1214,7 +1214,7 @@ Perl_isUTF8_CHAR(const U8 * const s0, const U8 * const e)
         return s - s0 + 1;
     }
 
-#if defined(UV_IS_QUAD) || defined(EBCDIC)
+#ifdef HAS_EXTRA_LONG_UTF8
 
     if (e - s0 >= UTF8_MAXBYTES && NATIVE_UTF8_TO_I8(*s0) == 0xFF) {
        return is_utf8_char_helper(s0, e, 0);

utf8.c

@@ -858,7 +858,7 @@ 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

@@ -360,6 +360,7 @@ C<cp> is Unicode if above 255; otherwise is platform-native.
 #if UVSIZE * CHARBITS > (6 * UTF_CONTINUATION_BYTE_INFO_BITS)
 #  define OFFUNISKIP_helper_(uv)                                            \
             UNLIKELY(uv > nBIT_UMAX(6 * UTF_CONTINUATION_BYTE_INFO_BITS))
+#  define HAS_EXTRA_LONG_UTF8
 #else
 #  define OFFUNISKIP_helper_(uv)  0
 #endif