<time.h>: Add timegm(), timelocal() and others.

Add timegm(), timelocal(), time2posix() and posix2time() to the
C library.

Change-Id: I34d5771ed83dd994870a5ca58a511d01898b1ffb
diff --git a/libc/bionic/time64_config.h b/libc/bionic/time64_config.h
index 53bcecf..1385045 100644
--- a/libc/bionic/time64_config.h
+++ b/libc/bionic/time64_config.h
@@ -33,7 +33,7 @@
 */
 #define HAS_GMTIME_R
 #define HAS_LOCALTIME_R
-/*#define HAS_TIMEGM */
+#define HAS_TIMEGM
 
 
 /* Details of non-standard tm struct elements.