boot/x86: if __uint128 is available, use it

Change-Id: I2b01b8e205ffcfb9f3901715a7c8382a21d4f575
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5140
Reviewed-by: Adrien Destugues <[email protected]>

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

@@ -79,6 +79,9 @@ uint32 gTimeConversionFactor;
 	// if the TSC just isn't stable and we can't get our desired error range.
 
 
+#ifdef __SIZEOF_INT128__
+typedef unsigned __int128 uint128;
+#else
 struct uint128 {
 	uint128(uint64 low, uint64 high = 0)
 		:
@@ -178,6 +181,7 @@ private:
 	uint64	low;
 	uint64	high;
 };
+#endif
 
 
 static inline void