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 adb19fb6 (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>