core: fix `coro_unwcontext` invalid unwind info

During the context switch required for backtracing a suspended fiber,
unwinders go crazy, as the unwind information they had gets implicitly
invalidated: provide an annotation for a dummy frame for
`coro_unwcontext`, as if it were at the bottom of the call-chain — that
way unwinders can normally proceed further.

We need to know the exact location of the stack pointer: replace the
16-byte stack alignment instruction on x86_64 macOS by adding the
`force_align_arg_pointer` attribute to `unw_getcontext_f`.

Needed for #4002

NO_DOC=bug fix
NO_CHANGELOG=bug fix
NO_TEST=unwind information annotation in inline assembly