core: eliminate dependency of core lib headers on libunwind.h

The only reason why libunwind.h is included into backtrace.h (and then
transitively into fiber.h) is use of unw_word_t type. Let's replace it
with uintptr_t and use unw_word_t only for interacting with the unwind
library.

This commit partially reverts commit 6d088b56 ("build: fix
libunwind.h search in test files"), because we don't need to
include libunwind.h into tests anymore.

Fixes #8025

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=changelogs/unreleased/fix-libunwind-h-search.md