selftests/bpf: Don't force lld on non-x86 architectures

selftests/bpf: Don't force lld on non-x86 architectures

LLVM's lld linker doesn't have a universal architecture support (e.g.,
it definitely doesn't work on s390x), so be safe and force lld for
urandom_read and liburandom_read.so only on x86 architectures.

This should fix s390x CI runs.

Fixes: 3e6fe5ce4d48 ("libbpf: Fix internal USDT address translation logic for shared libraries")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220617045512.1339795-1-andrii@kernel.org