arm64 toolchain: -mcpu=cortex-* branches are unreachable in both Toolchain.defs and cmake/Toolchain.cmake #20400

In arch/arm64/src/Toolchain.defs and arch/arm64/src/cmake/Toolchain.cmake the per-core -mcpu branches sit behind the CONFIG_ARCH_ARMV8A test, which always matches first because every ARCH_CORTEX_A5x config selects ARCH_ARMV8A (arch/arm64/Kconfig :332, :343, :354, :365):

• Toolchain.defs — ARCH_ARMV8A tested at :35, -mcpu=cortex-* branches at :51-58
• cmake/Toolchain.cmake — ARCH_ARMV8A tested at :28, same branches at :36-41

As a result the -mcpu=cortex-a53/a55/a57/a72 branches are unreachable in both build systems: every Cortex-A target builds with plain -march=armv8-a and loses per-core scheduling/tuning. The dead branches are also a trap for anyone adding a new core — a -mcpu entry appended at the end of the chain silently never takes effect.

Either move the -mcpu branches ahead of the CONFIG_ARCH_ARMV8A test so per-core flags win, or drop the dead branches. Happy to open a PR.