arm64: signal: split frame link record from sigcontext structure

In order to be able to increase the amount of the data currently
written to the __reserved[] array in the signal frame, it is
necessary to overwrite the locations currently occupied by the
{fp,lr} frame link record pushed at the top of the signal stack.

In order for this to work, this patch detaches the frame link
record from struct rt_sigframe and places it separately at the top
of the signal stack.  This will allow subsequent patches to insert
data between it and __reserved[].

This change relies on the non-ABI status of the placement of the
frame record with respect to struct sigframe: this status is
undocumented, but the placement is not declared or described in the
user headers, and known unwinder implementations (libgcc,
libunwind, gdb) appear not to rely on it.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>