[builtins] Use %% as the separator for aarch64-*-darwin and ; for powerpc{32,64}

[MaskRay]

Summary

; is the default comment marker, which is also used by powerpc*-*-elf target triples.
@ is the comment marker of powerpc*-*-darwin but the Darwin support has been deleted for PowerPC (D72063).
%% is the statement separator used by aarch64-*-darwin (see AArch64MCAsmInfoDarwin, it uses ; as the comment marker, which is different from most other targets)

[tambre]

Fixes the issue, thanks! I'll revert the workaround once this lands.

compiler-rt/lib/builtins/assembly.h

 //===-- assembly.h - compiler-rt assembler support macros -----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines macros for use in compiler-rt assembler source.
 // This file is not part of the interface of this library.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef COMPILERRT_ASSEMBLY_H
 #define COMPILERRT_ASSEMBLY_H
 
-#if defined(__POWERPC__) || defined(__powerpc__) || defined(__ppc__)
-#define SEPARATOR @
+#if defined(__APPLE__) && defined(__aarch64__)
+#define SEPARATOR %%
 #else
 #define SEPARATOR ;
 #endif
 
 #if defined(__APPLE__)
 #define HIDDEN(name) .private_extern name
 #define LOCAL_LABEL(name) L_##name
 // tell linker it can break up file at label boundaries