nplb test: Change stack allocation size from 4Mb to 1MB (#9030)

nplb test `PosixThreadAttrTest.StackAddrAndSizeAttr` was failing due to
large stack size allocation, possibly limitation on platform
implementation

Bug: 479846471
Change-Id: If502181c9aef3ce889a0309c68f83847a9699058

Author: santoshmahto7

starboard/nplb/posix_compliance/posix_thread_attr_test.cc

@@ -20,7 +20,7 @@
 namespace nplb {
 namespace {
 
-constexpr int kStackSize = 4 * 1024 * 1024;
+constexpr int kStackSize = 1 * 1024 * 1024;
 
 TEST(PosixThreadAttrTest, InitAttr) {
   pthread_attr_t attr;