Improve AMD k10temp and Vega20 gpu_metrics compatibility

[czakris]

This patch improves AMD hardware compatibility on older platforms.

Changes:

• Add k10temp fallback CPU temperature detection when Tdie/Tctl are unavailable.
• Add support for gpu_metrics v1.0 content_revision=0 used by older AMD GPUs such as Radeon VII (Vega20).
• Use hwmon temperature fallback when gpu_metrics exposes invalid temperature values (0xffff).

Tested on:

• AMD FX-8350 CPU (k10temp)
• AMD Radeon VII GPU (Vega20)
• Linux kernel amdgpu driver
• RADV Vulkan driver
• KDE Plasma on X11

The changes are backward compatible and keep existing newer gpu_metrics handling unchanged.

The patched build was installed using the standard Meson install process and verified to load correctly through the Vulkan implicit layer system.

On this older AMD platform MangoHud correctly reports:

• CPU temperature
• GPU temperature
• GPU power
• GPU core and memory clocks (GFXCLK/UCLK)