accel/tcg: Introduce tb_pc and log_pc

The availability of tb->pc will shortly be conditional.
Introduce accessor functions to minimize ifdefs.

Pass around a known pc to places like tcg_gen_code,
where the caller must already have the value.

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

target/openrisc/cpu.c

@@ -43,7 +43,7 @@ static void openrisc_cpu_synchronize_from_tb(CPUState *cs,
 {
     OpenRISCCPU *cpu = OPENRISC_CPU(cs);
 
-    cpu->env.pc = tb->pc;
+    cpu->env.pc = tb_pc(tb);
 }