[PAC][compiler-rt] Fix init/fini array signing schema (llvm#150691)

When `ptrauth_calls` is present but `ptrauth_init_fini` is not, compiler
emits raw unsigned pointers in `.init_array`/`.fini_array` sections.
Previously, `__do_init`/`__do_fini` pointers, which are explicitly added
to the sections, were implicitly signed (due to the presense of
`ptrauth_calls`), while all the other pointers in the sections were
implicitly added by the compiler and thus non-signed.. As a result, the
sections contained a mix of unsigned function pointers and function
pointers signed with default signing schema.

This patch introduces use of inline assembly for this particular case,
so we can manually specify that we do not want to sign the pointers.

Note that we cannot use `__builtin_ptrauth_strip` for this purpose since
its result is not a constant expression.

(cherry picked from commit 19ba224)

Author: kovdan01

compiler-rt/lib/builtins/crtbegin.c

@@ -54,22 +54,33 @@ static void __attribute__((used)) __do_init(void) {
 }
 
 #ifdef CRT_HAS_INITFINI_ARRAY
-#if __has_feature(ptrauth_init_fini)
+# if __has_feature(ptrauth_init_fini)
 // TODO: use __ptrauth-qualified pointers when they are supported on clang side
-#if __has_feature(ptrauth_init_fini_address_discrimination)
+#  if __has_feature(ptrauth_init_fini_address_discrimination)
 __attribute__((section(".init_array"), used)) static void *__init =
     ptrauth_sign_constant(&__do_init, ptrauth_key_init_fini_pointer,
                           ptrauth_blend_discriminator(
                               &__init, __ptrauth_init_fini_discriminator));
-#else
+#  else
 __attribute__((section(".init_array"), used)) static void *__init =
     ptrauth_sign_constant(&__do_init, ptrauth_key_init_fini_pointer,
                           __ptrauth_init_fini_discriminator);
-#endif
-#else
+#  endif
+# elif __has_feature(ptrauth_calls)
+#  ifdef __aarch64__
+// If ptrauth_init_fini feature is not present, compiler emits raw unsigned
+// pointers in .init_array. Use inline assembly to avoid implicit signing of
+// __do_init function pointer with ptrauth_calls enabled.
+__asm__(".pushsection .init_array,\"aw\",@init_array\n\t"
+        ".xword __do_init\n\t"
+        ".popsection");
+#  else
+#   error "ptrauth_calls is only supported for AArch64"
+#  endif
+# else
 __attribute__((section(".init_array"),
                used)) static void (*__init)(void) = __do_init;
-#endif
+# endif
 #elif defined(__i386__) || defined(__x86_64__)
 __asm__(".pushsection .init,\"ax\",@progbits\n\t"
         "call __do_init\n\t"
@@ -125,22 +136,33 @@ static void __attribute__((used)) __do_fini(void) {
 }
 
 #ifdef CRT_HAS_INITFINI_ARRAY
-#if __has_feature(ptrauth_init_fini)
+# if __has_feature(ptrauth_init_fini)
 // TODO: use __ptrauth-qualified pointers when they are supported on clang side
-#if __has_feature(ptrauth_init_fini_address_discrimination)
+#  if __has_feature(ptrauth_init_fini_address_discrimination)
 __attribute__((section(".fini_array"), used)) static void *__fini =
     ptrauth_sign_constant(&__do_fini, ptrauth_key_init_fini_pointer,
                           ptrauth_blend_discriminator(
                               &__fini, __ptrauth_init_fini_discriminator));
-#else
+#  else
 __attribute__((section(".fini_array"), used)) static void *__fini =
     ptrauth_sign_constant(&__do_fini, ptrauth_key_init_fini_pointer,
                           __ptrauth_init_fini_discriminator);
-#endif
-#else
+#  endif
+# elif __has_feature(ptrauth_calls)
+#  ifdef __aarch64__
+// If ptrauth_init_fini feature is not present, compiler emits raw unsigned
+// pointers in .fini_array. Use inline assembly to avoid implicit signing of
+// __do_fini function pointer with ptrauth_calls enabled.
+__asm__(".pushsection .fini_array,\"aw\",@fini_array\n\t"
+        ".xword __do_fini\n\t"
+        ".popsection");
+#  else
+#   error "ptrauth_calls is only supported for AArch64"
+#  endif
+# else
 __attribute__((section(".fini_array"),
                used)) static void (*__fini)(void) = __do_fini;
-#endif
+# endif
 #elif defined(__i386__) || defined(__x86_64__)
 __asm__(".pushsection .fini,\"ax\",@progbits\n\t"
         "call __do_fini\n\t"