get SYS_gettid value specific to given architecture (and hopefully fix the build for arm64)

Author: adamsitnik

src/libraries/System.Diagnostics.Process/tests/ProcessThreadTests.Unix.cs

@@ -32,7 +32,23 @@ public void TestPriorityLevelProperty_Unix()
             Assert.Throws<PlatformNotSupportedException>(() => thread.PriorityLevel = level);
         }
 
-        private static int GetCurrentThreadId() => syscall(186); // SYS_gettid
+        private static int GetCurrentThreadId()
+        {
+            // The magic values come from https://github.com/torvalds/linux.
+            int SYS_gettid = RuntimeInformation.ProcessArchitecture switch
+            {
+                Architecture.Arm => 224,
+                Architecture.Arm64 => 178,
+                Architecture.X86 => 224,
+                Architecture.X64 => 186,
+                Architecture.S390x => 236,
+                Architecture.Ppc64le => 207,
+                Architecture.RiscV64 => 178,
+                _ => 178,
+            };
+
+            return syscall(SYS_gettid);
+        }
 
         [DllImport("libc")]
         private static extern int syscall(int nr);