Fix Base::CpuCurrentFrequency on ARM/ARM64 Neoverse-N2 (Linux)

[ermig1979]

Summary

Base::CpuCurrentFrequency() returned 0 Hz on ARM/ARM64 Neoverse-N2 Linux.

Root causes addressed

1. Idle-core AMU zero: scaling_cur_freq can be 0 on the current core; old code returned immediately without trying other CPUs / max freq.
2. Round() overflow (prior revision): Round() returns int, so kHz * 1000 / MHz * 1e6 overflows around 2.1 GHz.
3. Missing ARM sources: no MHz in /proc/cpuinfo; need CPPC / SMBIOS / other fallbacks.

Fix (SimdBaseCpu.cpp)

1. Scan all cpu*/cpufreq + policy* files; take first value > 0 (kHz → Hz via 64-bit multiply)
2. ACPI CPPC nominal_freq / lowest_freq (MHz)
3. /proc/cpuinfo MHz
4. SMBIOS Type 4 from /sys/firmware/dmi/... (no root)
5. dmidecode / lscpu
6. Device-tree clock-frequency
7. perf_event_open cycle measurement (when permitted)

Also documents the fix in docs/2026.html release 7.2.165.

Test plan

• Build libSimd.so (g++)
• Frequency non-zero on this host (2400 MHz via /proc/cpuinfo)
• awk '$1 > 0' skips leading idle 0 readings and selects 2600000
• aarch64 cross-compile of SimdBaseCpu.cpp succeeds
• Retest on Neoverse-N2 with commit 97aff938f

If still 0 on Neoverse-N2, please share:

ls /sys/devices/system/cpu/cpu0/cpufreq /sys/devices/system/cpu/cpu0/acpi_cppc 2>&1
cat /sys/devices/system/cpu/cpu*/cpufreq/*cur* /sys/devices/system/cpu/cpu*/cpufreq/*max* 2>/dev/null | head
cat /sys/devices/system/cpu/cpu0/acpi_cppc/nominal_freq 2>&1
ls /sys/firmware/dmi/tables 2>&1
dmidecode -t processor 2>&1 | head -40
lscpu | head -40