Fix PowerPC restore_rt

* Clang fails to compile the CBE translation of this code ("non-ASM
  statement in naked function"). Similar to the implementations of
  `restore_rt` on x86 and ARM, when the CBE is in use, this commit
  employs alternative inline assembly that avoids using non-immediate
  input operands.
* As with all syscalls, the registers r0, r3, ctr, and xer should also
  be specified as clobbers.

Author: taylordotfish

lib/std/os/linux/powerpc.zig

@@ -193,11 +193,19 @@ pub fn clone() callconv(.naked) usize {
 pub const restore = restore_rt;
 
 pub fn restore_rt() callconv(.naked) noreturn {
-    asm volatile (
-        \\ sc
-        :
-        : [number] "{r0}" (@intFromEnum(SYS.rt_sigreturn)),
-        : .{ .memory = true, .cr0 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true });
+    switch (@import("builtin").zig_backend) {
+        .stage2_c => asm volatile (
+            \\ li 0, %[number]
+            \\ sc
+            :
+            : [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 (
+            \\ sc
+            :
+            : [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 }),
+    }
 }
 
 pub const F = struct {

lib/std/os/linux/powerpc64.zig

@@ -178,11 +178,19 @@ pub fn clone() callconv(.naked) usize {
 pub const restore = restore_rt;
 
 pub fn restore_rt() callconv(.naked) noreturn {
-    asm volatile (
-        \\ sc
-        :
-        : [number] "{r0}" (@intFromEnum(SYS.rt_sigreturn)),
-        : .{ .memory = true, .cr0 = true, .r4 = true, .r5 = true, .r6 = true, .r7 = true, .r8 = true, .r9 = true, .r10 = true, .r11 = true, .r12 = true });
+    switch (@import("builtin").zig_backend) {
+        .stage2_c => asm volatile (
+            \\ li 0, %[number]
+            \\ sc
+            :
+            : [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 (
+            \\ sc
+            :
+            : [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 }),
+    }
 }
 
 pub const F = struct {