Add the macro OPENSSL_SYS_WIN64

This is for consistency.
Additionally, have its presence define OPENSSL_SYS_WINDOWS as well.

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 3f13155)

Conflicts:
	e_os2.h

e_os2.h

@@ -109,6 +109,12 @@ extern "C" {
 #    undef OPENSSL_SYS_UNIX
 #    define OPENSSL_SYS_WIN32
 #   endif
+#   if defined(_WIN64) || defined(OPENSSL_SYSNAME_WIN64)
+#    undef OPENSSL_SYS_UNIX
+#    if !defined(OPENSSL_SYS_WIN64)
+#     define OPENSSL_SYS_WIN64
+#    endif
+#   endif
 #   if defined(OPENSSL_SYSNAME_WINNT)
 #    undef OPENSSL_SYS_UNIX
 #    define OPENSSL_SYS_WINNT
@@ -121,7 +127,7 @@ extern "C" {
 # endif
 
 /* Anything that tries to look like Microsoft is "Windows" */
-# if defined(OPENSSL_SYS_WIN32) || defined(OPENSSL_SYS_WINNT) || defined(OPENSSL_SYS_WINCE)
+# if defined(OPENSSL_SYS_WIN32) || defined(OPENSSL_SYS_WIN64) || defined(OPENSSL_SYS_WINNT) || defined(OPENSSL_SYS_WINCE)
 #  undef OPENSSL_SYS_UNIX
 #  define OPENSSL_SYS_WINDOWS
 #  ifndef OPENSSL_SYS_MSDOS