instrumentation: Avoid CPUID 0x15/0x16 for Hypervisor TSC frequency

This restricts the retrieval of the TSC frequency whilst under a Hypervisor to
either Hypervisor-specific CPUID registers (0x40000010), or TSC
calibration. We previously allowed retrieving from the traditional CPUID
registers for TSC frequency (0x15/0x16) like on bare metal, but it turns out
that they are not trustworthy when virtualized and can report wildly incorrect
frequencies, like 7 kHz when the actual calibrated frequencty is 2.5 GHz.

Per report from buildfarm member drongo.

Author: Lukas Fittl <lukas@fittl.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/jr4hk2sxhqcfpb67ftz5g4vw33nm67cgf7go3wwmqsafu5aclq%405m67ukuhyszz

src/port/pg_cpu_x86.c

@@ -165,19 +165,16 @@ x86_tsc_frequency_khz(void)
 {
 	unsigned int reg[4] = {0};
 
+	/*
+	 * If we're inside a virtual machine, try to fetch the TSC frequency from
+	 * the hypervisor, using a hypervisor specific method.
+	 *
+	 * Note it is not safe to utilize the regular 0x15/0x16 CPUID registers
+	 * (i.e. the logic below) in virtual machines, as they have been observed
+	 * to be wildly incorrect when virtualized.
+	 */
 	if (x86_feature_available(PG_HYPERVISOR))
-	{
+		return x86_hypervisor_tsc_frequency_khz();
-		uint32		freq = x86_hypervisor_tsc_frequency_khz();
-
-		/*
-		 * If the hypervisor specific logic didn't figure out the frequency,
-		 * it's possible (although not likely, as often that's hidden from
-		 * guests) that the non-virtualized logic can figure out the
-		 * frequency.
-		 */
-		if (freq > 0)
-			return freq;
-	}
 
 	/*
 	 * On modern Intel CPUs, the TSC is implemented by invariant timekeeping