Avoid dual input+clobber registers

As far as I can tell, this has only been a problem with the C backend
(see ziglang#25229), which is specifically excluded from this inline asm call.
However, it's probably best to avoid it anyway and use output registers
instead. In this case, because we're ignoring r3, which is the real
"output" of the syscall, we can specify r0 as the sole `->` output
instead, avoiding the need for a separate variable.

Author: taylordotfish

lib/std/os/linux/powerpc.zig

@@ -200,11 +200,11 @@ pub fn restore_rt() callconv(.naked) noreturn {
             :
             : [number] "i" (@intFromEnum(SYS.rt_sigreturn)),
             : .{ .memory = true, .cr0 = true, .r0 = true, .r3 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true, .ctr = true, .xer = true }),
-        else => asm volatile (
+        else => _ = asm volatile (
             \\ sc
-            :
+            : [r0] "={r0}" (-> usize), // r0 is clobbered
             : [number] "{r0}" (@intFromEnum(SYS.rt_sigreturn)),
-            : .{ .memory = true, .cr0 = true, .r0 = true, .r3 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true, .ctr = true, .xer = true }),
+            : .{ .memory = true, .cr0 = true, .r3 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true, .ctr = true, .xer = true }),
     }
 }
 

lib/std/os/linux/powerpc64.zig

@@ -185,11 +185,11 @@ pub fn restore_rt() callconv(.naked) noreturn {
             :
             : [number] "i" (@intFromEnum(SYS.rt_sigreturn)),
             : .{ .memory = true, .cr0 = true, .r0 = true, .r3 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true, .ctr = true, .xer = true }),
-        else => asm volatile (
+        else => _ = asm volatile (
             \\ sc
-            :
+            : [r0] "={r0}" (-> usize), // r0 is clobbered
             : [number] "{r0}" (@intFromEnum(SYS.rt_sigreturn)),
-            : .{ .memory = true, .cr0 = true, .r0 = true, .r3 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true, .ctr = true, .xer = true }),
+            : .{ .memory = true, .cr0 = true, .r3 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true, .ctr = true, .xer = true }),
     }
 }