Generalize tr/// compilation to work under EBCDIC

This changes to use a symbolic constant that is valid on any platform,
instead of a numeric value that isn't valid on EBCDIC platforms.

The value for this symbolic constant is unchanged on ASCII platforms,
which means this commit doesn't change any compiled code, so that it can
go into 5.42.

Author: khwilliamson

op.c

@@ -6410,9 +6410,9 @@ S_pmtrans(pTHX_ OP *o, OP *expr, OP *repl)
     UV PL_partition_by_byte_length[] = {
         0,
 
-        /* 0 .. 127  all have 1 byte
+        /* 0 .. 127 (0.. 159 on EBCDIC platforms) all have 1 byte
          * representations */
-        0x80,
+        UTF_MIN_CONTINUATION_BYTE,
 
         /* The highest two UTF-8 byte representable code point is the one with
          * all 1's in the payload bearing bits of the start byte and its single