coro: fix coro_{init,startup} unwind information

Fiber call-chains end at `coro_{init, startup}`, but unwinders don't
stop there, trying to use `coro_{init, startup}` stack frame's return
address (which points to some garbage) and, in turn, failing. A similar
issue was experienced by seastar and julia (see JuliaLang/julia#23074
and scylladb/scylladb#1909).

In order to make unwinding stop at `coro_{init, startup}`'s stack frame
we need to annotate it with CFI assembly: previously, annotation was
provided only for GCC on x86_64 — also provide it for clang.

For some reason unwinders ignore platform ABIs regarding ending of
call-chains: instead of trying to follow platform ABIs, explicitly
invalidate the topmost (`coro_{init, startup}`) stack frame information
for both x86_64 and AARCH64.

References:
1. glibc:
 * clone: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/
 sysv/linux/x86_64/clone.S;h=31ac12da0cc08a934d514fed1de9eba1cb3e8ec5;hb
 =ebbb8c9f64c3486603ef4ccee4dd2a5574e41039
 * start: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_6
 4/start.S;h=9edd17b60cd54ec9eef11c76ab02322dcb5d057a;hb=5b736bc9b55115e
 67129e77db4de6cf193054cd2
2. seastar:
 * thread_context::main(): https://github.com/scylladb/seastar/blob/d27b
 f8b5a14e5b9e9c9df18fd1306489b651aa42/src/core/thread.cc#L278-L293
3. julia:
 * https://github.com/JuliaLang/julia/blob/2e2b1d2ad50fe12999cbde
 d0b5acd3f0a36ec8c5/src/julia_internal.h#L90-L106
4. android:
 * https://cs.android.com/android/platform/superproject/+/master:bionic/
 libc/platform/bionic/macros.h;l=52-60;drc=2528dab7419a63f57fe20027886ba
 7dd3857aba8

Needed for tarantool#4002

NO_DOC=bug fix
NO_CHANGELOG=bug fix

third_party/coro/coro.c

@@ -103,9 +103,17 @@ coro_init (void)
 
   coro_transfer (new_coro, create_coro);
 
-#if __GCC_HAVE_DWARF2_CFI_ASM && __amd64
-  asm (".cfi_undefined rip");
-#endif
+  /* Call-chain ends here: we need to invalidate this frame's return
+   * address to make unwinding stop here.
+   * Some nuances on x86_64, see:
+   * https://github.com/libunwind/libunwind/blob/ec171c9ba7ea3abb2a1383cee2988a7
+   * abd483a1f/src/x86_64/Gstep.c#L91-L92
+   * https://github.com/libunwind/libunwind/blob/ec171c9ba7ea3abb2a1383cee2988a7
+   * abd483a1f/src/dwarf/Gparser.c#L877
+   */
+  __asm__(".cfi_undefined rip\n"
+	  ".cfi_undefined rbp\n"
+	  ".cfi_return_column rbp\n");
 
   func ((void *)arg);
 
@@ -320,13 +328,14 @@ trampoline (int sig)
          ".fnend\n"
 
        #elif __aarch64__
-
          ".cfi_startproc\n"
-         "\tmov x30, #0\n"
-         "\tsub sp, sp, #16\n"
-         "\tstr x30, [sp, #0]\n"
-         ".cfi_def_cfa_offset 16\n"
-         ".cfi_offset 30, -16\n"
+	 /*
+	  * Call-chain ends here: we need to invalidate this frame's return
+	  * address to make unwinding stop here.
+	  * No nuances: AARCH64 has a special link register for storing
+	  * return addresses.
+	  */
+	 ".cfi_undefined x30\n"
          "\tmov x0, x20\n"
          "\tblr x19\n"
 # ifdef __APPLE__
@@ -834,4 +843,3 @@ coro_stack_free (struct coro_stack *stack)
 }
 
 #endif
-

third_party/coro/coro.h

@@ -420,4 +420,3 @@ void coro_destroy (coro_context *ctx);
 #endif
 
 #endif
-