MIPS64: move fstat64/fstatat64/_flush_cache syscalls to SECCOMP_WHITELIST.TXT

fstat64/fstatat64/_flush_cache were accidentally put in SYSCALLS.TXT in:
    https://android-review.googlesource.com/#/c/390454/



This patch just moves them to SECCOMP_WHITELIST.TXT because we do not
want stubs accidenatally generated for the mentioned syscalls using
gensyscalls.py script.

This commit does not introduce any functional changes to mips64_policy.cpp.

Test: Run genseccomp.py -> File seccomp/mips64_policy.cpp not changed.
Test: Run gensyscalls.py -> INFO:root:no changes detected!

Change-Id: I3b527b3d9f18715c44a4e6ddc6db6e49f48f4890
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>