Use the larger stack size for ASan/UBSan builds on macOS, too

At least on my macOS 10.12.1, PTHREAD_STACK_MIN is 8192 (so stacksize was
substantially smaller than 12MB), and without this change there were randomly
looking failures in e.g. JunitTest_chart2_unoapi with a -fsanitize=address
build.

Change-Id: Icfe989a0e5097a9a0ae76c6e0f6ffcca18271245

sal/osl/unx/thread.cxx

@@ -277,10 +277,8 @@ static oslThread osl_thread_create_Impl (
 
 #if defined OPENBSD
     stacksize = 262144;
-#elif defined LINUX
-    stacksize = 12 * 1024 * 1024; // 8MB is not enough for ASAN on x86-64
 #else
-    stacksize = 100 * PTHREAD_STACK_MIN;
+    stacksize = 12 * 1024 * 1024; // 8MB is not enough for ASAN on x86-64
 #endif
     if (pthread_attr_setstacksize(&attr, stacksize) != 0) {
         pthread_attr_destroy(&attr);