libclang_rt_prebuilt_library_shared mixed builds

Also fix nil deref in setting $LOCAL_SOONG_UNSTRIPPED_BINARY

Bug: 201802518
Test: request_type_test.go
Test: prebuilt_test.go:TestPrebuiltLibrarySharedWithBazel
Test: mixed_{libc,droid}.sh
Change-Id: I22afb56c4b42d3412c2b2e1f079f1bcf8f3129a7