selftests/bpf: test_loader.c:get_current_arch() should not return 0

selftests/bpf: test_loader.c:get_current_arch() should not return 0

At the moment, when test_loader.c:get_current_arch() can't determine
the arch, it returns 0. The arch check in run_subtest() looks as
follows:

	if ((get_current_arch() & spec->arch_mask) == 0) {
		test__skip();
		return;
	}

Which means that all test_loader based tests would be skipped if arch
could not be determined. get_current_arch() recognizes x86_64, arm64
and riscv64. Which means that CI skips test_loader tests for s390.

Fix this by making sure that get_current_arch() always returns
non-zero value. In combination with default spec->arch_mask == -1 this
should cover all possibilities.

Fixes: f406026fefa7 ("selftests/bpf: by default use arch mask allowing all archs")
Fixes: 7d743e4c759c ("selftests/bpf: __jited test tag to check disassembly after jit")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240823080644.263943-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>