tcg/ppc: Reorg goto_tb implementation

tcg/ppc: Reorg goto_tb implementation

The old ppc64 implementation replaces 2 or 4 insns, which leaves a race
condition in which a thread could be stopped at a PC in the middle of
the sequence, and when restarted does not see the complete address
computation and branches to nowhere.

The new implemetation replaces only one insn, swapping between

	b       <dest>
and
	mtctr	r31

falling through to a general-case indirect branch.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>