tests: Use cc by default, not gcc.
Fix undefined-function error not working on #-sb-dynamic-core.

When compiled with clang on FreeBSD the code which looks for
undefined_tramp's location goes through dlsym, but clang doesn't
export runtime symbols by default.
Go through dlsym only when sb-dynamic-core is actually enabled.
Make sb-dynamic-core work on FreeBSD.
Fix small bug in FUNCTION-LAMBDA-EXPRESSION