alpha: enable regset-based ptrace and core dumps

Add a user_regset_view for Alpha and switch ELF core dumping to
CORE_DUMP_USE_REGSET.  General-purpose registers are exported in
ELF gregs layout, including callee-saved registers and a correct
user stack pointer.

The user stack pointer is not preserved in pt_regs on Alpha, so expose
it from the PCB, or via rdusp() for the current task, when building the
ELF register image.  This makes the user stack pointer consistent for
core dumps, ptrace regsets, and PTRACE_GET_SYSCALL_INFO.

Implement regset get/set callbacks for both NT_PRSTATUS and NT_PRFPREG.
The callbacks translate between Alpha's pt_regs/thread state and the
ELF-visible register layouts, while the common ptrace regset code handles
PTRACE_GETREGSET and PTRACE_SETREGSET iovec semantics.  This avoids
duplicating subtle short-buffer and oversized-buffer behavior in
arch_ptrace().

With these changes Alpha satisfies the requirements for
HAVE_ARCH_TRACEHOOK and selects it, enabling generic tracehook and
ptrace syscall-info code paths without changing the existing syscall
entry ABI.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20260706170019.2941459-2-linmag7@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

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