tcg: Define tcg_itype for code pointers

To be defined by the tcg backend based on the elemental unit of the ISA.
During the transition, allow TCG_TARGET_ITYPE_SIZE to be undefined, which
allows us to default tcg_itype to the current uint8_t.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-ppc64: Define TCG_TARGET_ITYPE_SIZE

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-ppc: Define TCG_TARGET_ITYPE_SIZE

Signed-off-by: Richard Henderson <rth@twiddle.net>