Add sys_ prefix to unwrapped system calls

This change also implements getlogin() and getlogin_r().

libc/intrin/sched_yield.S

@@ -28,6 +28,13 @@
 sched_yield:
 	push	%rbp
 	mov	%rsp,%rbp
+	xor	%eax,%eax
+	mov	__hostos(%rip),%dl
+
+#if SupportsMetal()
+	testb	$METAL,%dl
+	jnz	9f
+#endif
 
 #if SupportsWindows()
 //	Windows Support
@@ -39,7 +46,7 @@ sched_yield:
 //	threads ready to run and no user APCs are queued, the function
 //	returns immediately, and the thread continues execution.
 //	                                  ──Quoth MSDN
-	testb	IsWindows()
+	testb	$WINDOWS,%dl
 	jz	1f
 	xor	%ecx,%ecx
 	xor	%edx,%edx