This change enables the `--gc-sections` linker flag for WebAssembly
targets by default. This flag has been disabled because it did not
respect `llvm.used` and it caused stripping `swift5` metadata sections
even though they were used through encapsulation symbols (a.k.a `__start`/`__stop`).
The issue has been fixed in the LLVM side (ba3c1f9ce3)
by adding new segment flags to the WebAssembly object file format.
When we're statically linking the standard library, we need to force
the inclusion of the backtracing code in the runtime, otherwise we don't
get on-crash backtraces.
Also, add a test to make sure that this works.
rdar://115774613
