vdso: don't require 64-bit math in standalone test

The use of 64-bit math on i386 causes build failures:
vdso_standalone_test_x86.c:(.text+0x101): undefined reference to `__umoddi3'
vdso_standalone_test_x86.c:(.text+0x12d): undefined reference to `__udivdi3'

Commit adb19fb66ee (Documentation: add makefiles for more targets) is
now building this by default, so it's failing the kernel build entirely.

Switching the declaration from uint64_t to time_t does the right thing
and handles the x32 case automatically.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Acked-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vDSO/vdso_standalone_test_x86.c

@@ -63,7 +63,7 @@ static inline void linux_exit(int code)
 	x86_syscall3(__NR_exit, code, 0, 0);
 }
 
-void to_base10(char *lastdig, uint64_t n)
+void to_base10(char *lastdig, time_t n)
 {
 	while (n) {
 		*lastdig = (n % 10) + '0';