Use local label in naked function to avoid collisions

This status-quo ante of this naked function test defined a label with
the symbolic name `.L7`.  However, changes to the stack could cause a
collision with that same symbolic name being defined by compiler-
generated code; therefore we use a local label instead to guarantee
collision avoidance.

Author: eggyal

tests/ui/c-variadic/naked.rs

@@ -15,9 +15,9 @@ unsafe extern "sysv64" fn c_variadic_sysv64(_: ...) -> Data {
         "        sub     rsp, 96",
         "        mov     QWORD PTR [rsp-88], rdi",
         "        test    al, al",
-        "        je      .L7",
+        "        je      0f",
         "        movaps  XMMWORD PTR [rsp-40], xmm0",
-        ".L7:",
+        "0:",
         "        lea     rax, [rsp+104]",
         "        mov     rcx, QWORD PTR [rsp-40]",
         "        mov     DWORD PTR [rsp-112], 0",