kernel: add Kconfig options for ARM64_BRBE and THP enhancements on 6.18

kernel: add Kconfig options for ARM64_BRBE and THP enhancements on 6.18

Introduce new kernel configuration options for memory management
and hardware-profiling features:

- KERNEL_ARM64_BRBE: Enables support for the Branch Record Buffer
  Extension (FEAT_BRBE) on ARM64 architectures when perf events are active.
- KERNEL_PERSISTENT_HUGE_ZERO_FOLIO: Allows allocating a PMD-sized
  folio for zeroing, optimizing Transparent Hugepage (THP) allocation
  performance.
- KERNEL_NO_PAGE_MAPCOUNT: Adds experimental support for bypassing
  per-page mapcount tracking to reduce memory accounting overhead under THP.

These options map directly to their respective kernel options and are
applicable on 6.18 and later.

Signed-off-by: Joshua Covington <joshuacov@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23703
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>