arm64: split syscall_trace() into separate functions for enter/exit

As done in arm, this change makes it easy to confirm we invoke syscall
related hooks, including syscall tracepoint, audit and seccomp which would
be implemented later, in correct order. That is, undoing operations in the
opposite order on exit that they were done on entry.

Change-Id: Ie5ea4e5f517ccd1e47061be0e2144a7e1556f31a
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 3157858feff89196635b01495d5ec9ebe206639e
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Signed-off-by: Pratik Patel <pratikp@codeaurora.org>