arch: riscv: fix hangup in boot if hart0 is not boot hart

This patch changes the section of riscv_cpu_wake_flag variable to
noinit from bss to fix hangup of RISC-V multicore boot if hart0 is
not boot hart (CONFIG_RV_BOOT_HART != 0).

Current boot sequence initializes a riscv_cpu_wake_flag to -1 but
this variable is unintentionally changed to 0 by boot hart.
This is because the variable is placed in bss section so this patch
changes the section of the variable to noinit.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>