[libc][NFC] Simplify AMDGPU constant frequency checks (#79653)

Summary:
The AMDGPU fixed frequency clock is fixed to a chip dependent frequency.
More modern chips have started to fix this at known values of 25 MHz or
100 MHz, so this function forwards those values. This was done using the
individual architectures. This patch simply uses the more concise
`__GFXn__` macro which indicates the major revision