target/mips: Declare mips_env_set_pc() inlined in "internal.h"

Rename set_pc() as mips_env_set_pc(), declare it inlined
and use it in cpu.c and op_helper.c.

Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210428170410.479308-9-f4bug@amsat.org>