tcg/ppc: Enable direct branching tcg_out_goto_tb with TCG_REG_TB

tcg/ppc: Enable direct branching tcg_out_goto_tb with TCG_REG_TB

Direct branch patching was disabled when using TCG_REG_TB in commit
736a1588c1 ("tcg/ppc: Fix race in goto_tb implementation").

The issue with direct branch patching with TCG_REG_TB is the lack of
synchronization between the new TCG_REG_TB being established and the
direct branch being patched in.

If each translation block is responsible for establishing its own
TCG_REG_TB then there can be no synchronization issue.

Make each translation block begin by setting up its own TCG_REG_TB.
Use the preferred 'bcl 20,31,$+4' sequence.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[rth: Split out tcg_out_tb_start, power9 addpcis]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>