On 32-bit IRQ_STACK_SIZE is the same as THREAD_SIZE.

To allow sharing struct irq_stack with 32-bit, define IRQ_STACK_SIZE for
32-bit and use it for struct irq_stack.

No functional change.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190414160145.632513987@linutronix.de