kselftest/coredump: reintroduce null pointer dereference

Commit 673a55cc49da replaced the null pointer dereference used in
crashing_child() with __builtin_trap to address the following LLVM warnings:

coredump_test_helpers.c:59:6: warning: indirection of non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
coredump_test_helpers.c:59:6: note: consider using __builtin_trap() or qualifying pointer with 'volatile'

All coredump tests expect crashing_child() to result in a SIGSEGV. However, the
behavior of __builtin_trap is architecture-dependent. On x86 it yields SIGILL,
on aarch64 SIGTRAP. Given that neither of those signals are SIGSEGV, both
coredump_socket_test and coredump_socket_protocol_test are currently failing:

get_pidfd_info: mask=0xd7, coredump_mask=0x5, coredump_signal=5
socket_coredump_signal_sigsegv: coredump_signal=5, expected SIGSEGV=11

Qualify the pointer with volatile instead of calling __builtin_trap to fix the
tests.

Signed-off-by: Emanuele Rocca <emanuele.rocca@arm.com>
Link: https://patch.msgid.link/ab2kI0PI_Vk6bU88@NH27D9T0LF
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>