lkl: decrease stack size of thread_create to PTHREAD_STACK_MIN

By default, each pthread has 8MB stack. LKL creates 20+ threads at
startup which consumes 160MB alone.

This commit decreases it to PTHREAD_STACK_MIN (16 KB) which is enough to
run linux kernel code. kernel stack on x86_64 is 2 pages (8 KB).

Tested:
MALLOC_ARENA_MAX=1 ./bin/lkl-hijack.sh sleep 1000
pmap virt mem: 287MB vs 115 MB

Signed-off-by: Yuan Liu <liuyuan@google.com>

tools/lkl/lib/posix-host.c

@@ -14,6 +14,7 @@
 #include <sys/socket.h>
 #include <sys/syscall.h>
 #include <poll.h>
+#include <limits.h>
 #include <lkl_host.h>
 #include "iomem.h"
 
@@ -177,7 +178,10 @@ static void mutex_free(struct lkl_mutex *_mutex)
 static lkl_thread_t thread_create(void (*fn)(void *), void *arg)
 {
 	pthread_t thread;
-	if (WARN_PTHREAD(pthread_create(&thread, NULL, (void* (*)(void *))fn, arg)))
+	pthread_attr_t attr;
+	pthread_attr_init(&attr);
+	pthread_attr_setstacksize(&attr, PTHREAD_STACK_MIN);
+	if (WARN_PTHREAD(pthread_create(&thread, &attr, (void* (*)(void *))fn, arg)))
 		return 0;
 	else
 		return (lkl_thread_t) thread;