selftests/bpf: Fix arena_spin_lock compilation on PowerPC

selftests/bpf: Fix arena_spin_lock compilation on PowerPC

Venkat reported a compilation error for BPF selftests on PowerPC [0].
The crux of the error is the following message:
  In file included from progs/arena_spin_lock.c:7:
  /root/bpf-next/tools/testing/selftests/bpf/bpf_arena_spin_lock.h:122:8:
  error: member reference base type '__attribute__((address_space(1)))
  u32' (aka '__attribute__((address_space(1))) unsigned int') is not a
  structure or union
     122 |         old = atomic_read(&lock->val);

This is because PowerPC overrides the qspinlock type changing the
lock->val member's type from atomic_t to u32.

To remedy this, import the asm-generic version in the arena spin lock
header, name it __qspinlock (since it's aliased to arena_spinlock_t, the
actual name hardly matters), and adjust the selftest to not depend on
the type in vmlinux.h.

  [0]: https://lore.kernel.org/bpf/7bc80a3b-d708-4735-aa3b-6a8c21720f9d@linux.ibm.com

Fixes: 88d706ba7cc5 ("selftests/bpf: Introduce arena spin lock")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Link: https://lore.kernel.org/bpf/20250311154244.3775505-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>