Dynamically allocate the alternate signal stack (ocaml#10266)

In Glibc 2.34 and later, SIGSTKSZ may not be a compile-time constant.
It is no longer possible to statically allocate the alternate signal
stack for the main thread, as we've been doing for the last 25 years.

This commit implements dynamic allocation of the alternate signal stack.

Fixes: ocaml#10250.

runtime/signals_nat.c

@@ -182,7 +182,6 @@ DECLARE_SIGNAL_HANDLER(trap_handler)
 #ifdef HAS_STACK_OVERFLOW_DETECTION
 
 static char * system_stack_top;
-static char sig_alt_stack[SIGSTKSZ];
 
 #if defined(SYS_linux)
 /* PR#4746: recent Linux kernels with support for stack randomization
@@ -275,7 +274,15 @@ void caml_init_signals(void)
   {
     stack_t stk;
     struct sigaction act;
-    stk.ss_sp = sig_alt_stack;
+    stk.ss_sp = malloc(SIGSTKSZ);
+    /* Allocate and select an alternate stack for handling signals,
+       especially SIGSEGV signals.
+       Each thread needs its own alternate stack.
+       The alternate stack used to be statically-allocated for the main thread,
+       but this is incompatible with Glibc 2.34 and newer, where SIGSTKSZ
+       may not be a compile-time constant (issue #10250). */
+    if (stk.ss_sp == NULL)
+      return;
     stk.ss_size = SIGSTKSZ;
     stk.ss_flags = 0;
     SET_SIGACT(act, segv_handler);

tools/ci/inria/lsan-suppr.txt

@@ -1,2 +1,4 @@
 # ocamlyacc doesn't clean memory on exit
 leak:ocamlyacc
+# Alternate signal stacks are currently never freed (see #10266)
+leak:caml_setup_stack_overflow_detection