[common] Fix RDTSC nanosecond conversion (#4422)

Split the RDTSC ticks-per-nanosecond ratio into integer and Q0.64 fractional components. This avoids overflowing GetFixedPoint64Factor for TSC frequencies above 1 GHz while preserving the multiply-only conversion path.

Use the invariant clock only above 1 GHz because the reverse nanoseconds-per-tick factor cannot represent an exact ratio of one.

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-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4422
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/common/cpu_features.h

@@ -96,6 +96,7 @@ public:
     u64 ns_rdtsc_factor;
     u64 us_rdtsc_factor;
     u64 ms_rdtsc_factor;
+    u64 rdtsc_ns_integer;
     u64 rdtsc_ns_factor;
     u64 cntpct_rdtsc_factor;
     u64 gputick_rdtsc_factor;