A new shared header tcg-pool.inc.c adds new_pool_label,
for registering a tcg_target_ulong to be emitted after
the generated code, plus relocation data to install a
pointer to the data.

A new pointer is added to the TCGContext, so that we
dump the constant pool as data, not code.
Signed-off-by: NRichard Henderson <rth@twiddle.net>