Sandbox libbssl_sys_src_nostd

go/roboleaf-busy-beavers-sandboxing

gensrcs should require output_extension to be set, when it's not,
you get some weird filename like `lib.`. Switch to genrules for
simplicity.

Bug: 307824623
Test: ./build/soong/tests/genrule_sandbox_test.py libbssl_sys_src_nostd
Change-Id: I4ec2686c560439c3150b74b14e313ed6b688720c