ARM64: fix KSTK_ESP

If a thread is compat, then stack start address is stored in
compat_sp not sp of pt_regs.

Bug 200006667

Change-Id: I4814d4b2a39224e0541ce988f84e038bca7d57bf
Signed-off-by: Kerwin Wan <kerwinw@nvidia.com>
Reviewed-on: http://git-master/r/419378


Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Krishna Reddy <vdumpa@nvidia.com>
Reviewed-by: Varun Wadekar <vwadekar@nvidia.com>
Git-repo: git://codeaurora.org/quic/la/kernel/tegra.git


Git-commit: cc0d4317057d190d465e1890c2aed29aef84762d
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>