PERL_HASH() casting games so that our hashed data is "unsigned

PERL_HASH() casting games so that our hashed data is "unsigned
char" but old code using just a "char" doesn't need changes.
(The change is using a temporary pointer instead of a direct
cast to unsigned char* which would blindly cast anything,
not just char pointers.)  (The problem arose in MacOS Classic,
as seen by Pudge, the cure by Nicholas Clark.)

p4raw-id: //depot/perl@16656