Fix system_time implementation

When refactoring this and writing a common implementation for x86 and
x86_64 I missed a step in the calculation. We need to divide by 2^32 to
keep the value in the expected range.

Fixes #15658.

src/system/boot/arch/x86/cpu.cpp

@@ -304,7 +304,8 @@ slower_sample:
 extern "C" bigtime_t
 system_time()
 {
-	return rdtsc() * gTimeConversionFactor;
+	uint128 tsc(rdtsc());
+	return uint64((tsc * gTimeConversionFactor) >> 32);
 }