arm64: Add 32-bit sigcontext definition to uapi signcontext.h

The arm64 uapi sigcontext.h can be included by 32-bit userspace
modules.  Since arm and arm64 sigcontext definition are not
compatible, add arm sigcontext definition to arm64 sigcontext.h.

Signed-off-by: David Ng <dave@codeaurora.org>
Signed-off-by: Divya Sharma <c_shard@codeaurora.org>
Signed-off-by: Anh Nguyen <anguyen@codeaurora.org>
Change-Id: Iaed58045fd345e4ca66e8a6981c6b4f80c4bdfca