fixes/cleanup for use-system-unwind make flags (#55639)

Assuming non-windows and libunwind not disabled:

The flag `-DLLVMLIBUNWIND` is currently set on macos only for
`USE_SYSTEM_UNWIND=0` which seems wrong to me and causes build issues
for macos on Yggdrasil in combination with the recent
#55049 which should only affect
gnu libunwind (`error: call to undeclared function 'unw_ensure_tls'`).
This flag is now set independently of the system-libunwind flag (on
Darwin and OpenBSD as before).

`LIBUNWIND=-lunwind` is set for `USE_SYSTEM_UNWIND=0` ||
`USE_SYSTEM_UNWIND=1` && `OS != Darwin`.
I don't think the check for Darwin make sense and might be a leftover
from using osxunwind a (long) while ago.
Changed that to always set `-lunwind` if enabled.

x-ref: JuliaPackaging/Yggdrasil#9331

Make.inc

@@ -1095,20 +1095,13 @@ LIBUNWIND:=
 else ifneq ($(DISABLE_LIBUNWIND), 0)
 LIBUNWIND:=
 else
-ifeq ($(USE_SYSTEM_LIBUNWIND), 1)
-ifneq ($(OS),Darwin)
 LIBUNWIND:=-lunwind
-# Only for linux since we want to use not yet released libunwind features
-JCFLAGS+=-DSYSTEM_LIBUNWIND
-JCPPFLAGS+=-DSYSTEM_LIBUNWIND
-endif
-else
 ifneq ($(findstring $(OS),Darwin OpenBSD),)
-LIBUNWIND:=-lunwind
 JCPPFLAGS+=-DLLVMLIBUNWIND
-else
-LIBUNWIND:=-lunwind
-endif
+else ifeq ($(USE_SYSTEM_LIBUNWIND), 1)
+# Only for linux and freebsd since we want to use not yet released gnu libunwind features
+JCFLAGS+=-DSYSTEM_LIBUNWIND
+JCPPFLAGS+=-DSYSTEM_LIBUNWIND
 endif
 endif