ftrace: Make ftrace_regs abstract from direct use

ftrace_regs was created to hold registers that store information to save
function parameters, return value and stack. Since it is a subset of
pt_regs, it should only be used by its accessor functions. But because
pt_regs can easily be taken from ftrace_regs (on most archs), it is
tempting to use it directly. But when running on other architectures, it
may fail to build or worse, build but crash the kernel!

Instead, make struct ftrace_regs an empty structure and have the
architectures define __arch_ftrace_regs and all the accessor functions
will typecast to it to get to the actual fields. This will help avoid
usage of ftrace_regs directly.

Link: https://lore.kernel.org/all/[email protected]/

Cc: "[email protected]" <[email protected]>
Cc: "[email protected]" <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: WANG Xuerui <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Naveen N Rao <[email protected]>
Cc: Madhavan Srinivasan <[email protected]>
Cc: Paul  Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Thomas  Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav  Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Link: https://lore.kernel.org/[email protected]
Acked-by: Catalin Marinas <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>
Acked-by: Masami Hiramatsu (Google) <[email protected]>
Acked-by: Heiko Carstens <[email protected]> # s390
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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