Allowlist //system/apex.

This bp2builds the one cc_binary in that package that uses
library_linking_strategy_cc_defaults. That target is buildable, but
there's still a bp2build_available allowlist bug that causes the select
statements and static_libs deps to not show up for the target (only
dynamic_deps are), due to how bp2build_available is special cased for
soong_config_module_type in the loadhook.

Test: b build //system/apex:library_linking_strategy_sample_binary
Change-Id: Ic38a1bfaa476108db63348d030e196d8561300ca