tcg/ppc: Optimize 26-bit jumps

PowerPC64 processors handle direct branches better than indirect
ones, resulting in less stalled cycles and branch misses.

However, PPC's tb_target_set_jmp_target() was only using direct
branches for 16-bit jumps, while PowerPC64's unconditional branch
instructions are able to handle displacements of up to 26 bits.
To take advantage of this, now jumps whose displacements fit in
between 17 and 26 bits are also converted to direct branches.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Leandro Lupori <leandro.lupori@eldorado.org.br>
[rth: Expanded some commentary.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>