str_hash_find_int

This changes the internal representation of string_tuple.value from
a void* to an intptr_t, removing any concerns that code wanting to
store an integer value will use values that are trap encodings or
suchlike for a pointer.  The ISO C standard says any void* can be
converted to intptr_t and back again and will compare equal to the
original pointer.  It does *not* say any intptr_t can be converted to
void* and back again to get the original integer..

Two new functions, str_hash_find_int and str_hash_insert_int are
provided for handling integer values.  str_hash_find_int returns
(intptr_t) -1 on failing to find the key string.

Most target code need minimal changes to use the new interface, but
some simplification is possible since now a zero can be stored and
differentiated from the NULL "can't find" return.  (Yes, that means
(intptr_t) -1 can't be stored.)

I've changed the avr_no_sreg_hash dummy value to zero, and the
loongarch register numbers don't need to be incremented.  loongarch
also doesn't need to store an empty key string (if it ever did).