tcg: Add TCGV_UNUSED_PTR, TCGV_IS_UNUSED_PTR, TCGV_EQUAL_PTR

We have macros for marking TCGv values as unused, checking if they
are unused and comparing them to each other. However these only exist
for TCGv_i32 and TCGv_i64; add them for TCGv_ptr as well.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
