main/linux-lts: Increase CONFIG_LOG_BUF_SHIFT for 64-bit archs
16 KiB is very small for modern machines. This increases it to the upstream's current default (128 KiB), but only for 64-bit architectures.
Alternatively, I guess we could just remove our config for this
variable, and let it take on the upstream's default. That's
already the case for e.g. lts.ppc64le.config
currently.