In Wasm EH, even if we set `-sASSERTION` or `-sEXCEPTION_STACK_TRACES`,
if we rethrow an exception in the code, we lose the effect of that
option because previously we called `__throw_exception_with_stack_trace`
only in `__cxa_throw`:
https://github.com/emscripten-core/emscripten/blob/9ce7020632aa6f7578c6e40e197b800a4dd7073f/system/lib/libcxxabi/src/cxa_exception.cpp#L294-L296

This adds the same mechanism to `__cxa_rethrow` (which is used for
C++ `throw;`) and `__cxa_rethrow_primary_exception` (which is used for
`std::rethrow_exception`).

This does not solve the problem of losing stack traces _before_ the
rethrowing. libc++abi's `__cxa_rethrow` and
`__cxa_rethrow_primary_exception` are implemented as throwing a pointer
in the same way we first throw it and they are not aware of any
metadata. This happens even in the native platform with GDB; GDB's
backtrace only shows stack traces after rethrowing. We may try to fix
this later by passing `exnref`
(WebAssembly/exception-handling#281) to the
library, but this is likely to take some time.

Partially fixes emscripten-core#20301.