Renumber the low PIKE_T_* values so that PIKE_T_INT becomes zero.

This has the feature that zeroed memory becomes filled with Pike
svalues containing integer zeroes (and not NULL pointer arrays).
This will let call_c_initializers() avoid traversing the entire
identifier table for the class.

Note: The serialized representation of types (__parse_pike_type())
      is unchanged. As is the {out,in}put for {en,de}code_value().

Updates the code generators for ia32 and amd64.

Breaks the code generators for ppc32 and ppc64.