arm64: compat: Provide definition for COMPAT_SIGMINSTKSZ

BugLink: https://bugs.launchpad.net/bugs/1844155

arch/arm/ defines a SIGMINSTKSZ of 2k, so we should use the same value
for compat tasks.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Reported-by: Steve McIntyre <steve.mcintyre@arm.com>
Tested-by: Steve McIntyre <93sam@debian.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

(cherry picked from commit 24951465)
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>

arch/arm64/include/asm/compat.h

@@ -234,6 +234,7 @@ static inline compat_uptr_t ptr_to_compat(void __user *uptr)
 }
 
 #define compat_user_stack_pointer() (user_stack_pointer(task_pt_regs(current)))
+#define COMPAT_MINSIGSTKSZ	2048
 
 static inline void __user *arch_compat_alloc_user_space(long len)
 {