rcu: Fix undefined Kconfig macros

Invoking scripts/checkkconfigsymbols.py in the Linux-kernel source tree
located the following issues:

1. TREE_PREEMPT_RCU
Referencing files: arch/sh/configs/sdk7786_defconfig

It should now be CONFIG_PREEMPT_RCU. Except that the CONFIG_PREEMPT=y in
that same file implies CONFIG_PREEMPT_RCU=y.  Therefore, delete the
CONFIG_TREE_PREEMPT_RCU=y line.

The reason is as follows:

In kernel/rcu/Kconfig, we have
config PREEMPT_RCU
        bool
        default y if PREEMPTION

https://www.kernel.org/doc/Documentation/kbuild/kconfig-language.txt says,
"The default value is only assigned to the config symbol if no other value
 was set by the user (via the input prompt above)."
there is no prompt in config PREEMPT_RCU entry, so we are guaranteed to
get CONFIG_PREEMPT_RCU=y when CONFIG_PREEMPT is present.

2. RCU_CPU_STALL_INFO
Referencing files: arch/xtensa/configs/nommu_kc705_defconfig

The old Kconfig option RCU_CPU_STALL_INFO was removed by commit
75c27f11 ("rcu: Remove CONFI...