tcg: Split out tcg_region_prologue_set

tcg: Split out tcg_region_prologue_set

This has only one user, but will make more sense after some
code motion.

Always leave the tcg_init_ctx initialized to the first region,
in preparation for tcg_prologue_init().  This also requires
that we don't re-allocate the region for the first cpu, lest
we hit the assertion for total number of regions allocated .

Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>