libunwind: requires glibc if arch in powerpc
libunwind for powerpc depends on getcontext() from libc which musl-libc
does not provide because this API and its friends are supposed to be
"obsolescent" [1,2]
[1] Subject: Re: setcontext/getcontext/makecontext missing?
https://www.openwall.com/lists/musl/2016/02/04/5
[2] http://pubs.opengroup.org/onlinepubs/
009695399/functions/makecontext.html
Refs: https://github.com/openwrt/packages/issues/8548#issuecomment-
497200058
Reported-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>