syscall_test: Do not test st_atime in the test for fstat
syscall_test: Do not test st_atime in the test for fstat

While POSIX says st_atime is updated only when the contents
of a file is read/modified, it seems Windows XP updates it
even when we only access the metadata of a file.

TEST=./scons run_syscall_test_irt run_irt_futex_test_irt \
     --mode=dbg-host,nacl_irt_test bitcode=1 nonsfi_nacl=1
BUG=None
TBR=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/387433003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@13464 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client