Fix newlib/testsuite/newlib.time/tzset.c compilation for 16-bit targets

When the test case is compiled for a target with a 16-bit int, the compiler
emits the two warnings shown below, causing the test to fail. The code
assumes that int is 32 bits wide, which is not always the case. This patch
removes that assumption and fixes the resulting compilation warnings.

Signed-off-by: Jan Dubiec <jdx@o2.pl>

Author: jdx-gh

newlib/testsuite/newlib.time/tzset.c

@@ -1,18 +1,19 @@
 /* Test that valid POSIX timezone strings are correctly parsed by tzset(3). */
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdint.h>
 
 // BEGIN test vectors
 #include <time.h>
 #include <limits.h>
 
-#define IN_SECONDS(h, m, s) ((h) * 3600 + (m) * 60 + (s))
+#define IN_SECONDS(h, m, s) ((h) * INT32_C(3600) + (m) * INT32_C(60) + (s))
 #define NO_TIME INT_MIN
 
 struct tz_test {
     const char* tzstr;
-    int offset_seconds;
-    int dst_offset_seconds;
+    int32_t offset_seconds;
+    int32_t dst_offset_seconds;
 };
 
 extern struct tm winter_tm;