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.

Change-Id: I94109b094f6c8376fdaeb2822d7b26d18ddfb2bc
Signed-off-by: David Ng <dave@codeaurora.org>
Signed-off-by: Natiq Ahmed <mnatiq@codeaurora.org>