selftests/bpf: Fix cross-compiling urandom_read

selftests/bpf: Fix cross-compiling urandom_read

Linking of urandom_read and liburandom_read.so prefers LLVM's 'ld.lld' but
falls back to using 'ld' if unsupported. However, this fallback discards
any existing makefile macro for LD and can break cross-compilation.

Fix by changing the fallback to use the target linker $(LD), passed via
'-fuse-ld=' using an absolute path rather than a linker "flavour".

Fixes: 08c79c9cd67f ("selftests/bpf: Don't force lld on non-x86 architectures")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241009040720.635260-1-tony.ambardar@gmail.com