Revert "remove plan9 test coverage": Plan9 now works :^)

This reverts commit 67db2b85b771d8b0f9d765922951c31fce9c8cc2.

test/cases/plan9/exit.zig

@@ -0,0 +1,5 @@
+pub fn main() void {}
+
+// run
+// target=x86_64-plan9,aarch64-plan9
+//

test/cases/plan9/hello_world_with_updates.0.zig

@@ -0,0 +1,28 @@
+pub fn main() void {
+    const str = "Hello World!\n";
+    asm volatile (
+        \\push $0
+        \\push %%r10
+        \\push %%r11
+        \\push $1
+        \\push $0
+        \\syscall
+        \\pop %%r11
+        \\pop %%r11
+        \\pop %%r11
+        \\pop %%r11
+        \\pop %%r11
+        :
+        // pwrite
+        : [syscall_number] "{rbp}" (51),
+          [hey] "{r11}" (@ptrToInt(str)),
+          [strlen] "{r10}" (str.len),
+        : "rcx", "rbp", "r11", "memory"
+    );
+}
+
+// run
+// target=x86_64-plan9
+//
+// Hello World
+//

test/cases/plan9/hello_world_with_updates.1.zig

@@ -0,0 +1,10 @@
+const std = @import("std");
+pub fn main() void {
+    const str = "Hello World!\n";
+    _ = std.os.plan9.pwrite(1, str, str.len, 0);
+}
+
+// run
+//
+// Hello World
+//