unix/mpthreadport: Ensure enough thread stack to detect overflow.

Following up to 5e6cee07ab, some systems (eg
FreeBSD 12.0 64-bit) will crash if the stack-overflow margin is too small.
It seems the margin of 8192 bytes (or thereabouts) is always needed.  This
commit adds this much margin if the requested stack size is too small.

Fixes issue #5824.

ports/unix/mpthreadport.c

@@ -47,6 +47,9 @@
 #define MP_THREAD_GC_SIGNAL (SIGUSR1)
 #endif
 
+// This value seems to be about right for both 32-bit and 64-bit builds.
+#define THREAD_STACK_OVERFLOW_MARGIN (8192)
+
 // this structure forms a linked list, one node per active thread
 typedef struct _thread_t {
     pthread_t id;           // system id of thread
@@ -193,6 +196,11 @@ void mp_thread_create(void *(*entry)(void *), void *arg, size_t *stack_size) {
         *stack_size = PTHREAD_STACK_MIN;
     }
 
+    // ensure there is enough stack to include a stack-overflow margin
+    if (*stack_size < 2 * THREAD_STACK_OVERFLOW_MARGIN) {
+        *stack_size = 2 * THREAD_STACK_OVERFLOW_MARGIN;
+    }
+
     // set thread attributes
     pthread_attr_t attr;
     int ret = pthread_attr_init(&attr);
@@ -220,12 +228,7 @@ void mp_thread_create(void *(*entry)(void *), void *arg, size_t *stack_size) {
     }
 
     // adjust stack_size to provide room to recover from hitting the limit
-    // this value seems to be about right for both 32-bit and 64-bit builds
-    if (*stack_size >= 2 * 8192) {
-        *stack_size -= 8192;
-    } else {
-        *stack_size /= 2;
-    }
+    *stack_size -= THREAD_STACK_OVERFLOW_MARGIN;
 
     // add thread to linked list of all threads
     thread_t *th = malloc(sizeof(thread_t));