makecontext, swapcontext, … are not provided by musl itself. Instead, we
depend on libucontext for these functions. plan9port uses them in
libthread, we need to link against -lucontext in order to make libthread
use them properly.

diff -upr a/bin/9l b/bin/9l
--- a/bin/9l	2020-12-15 06:06:03.000000000 +0100
+++ b/bin/9l	2020-12-23 14:43:20.376181908 +0100
@@ -25,7 +25,7 @@ case "$tag" in
 *Linux*)
 	ld="${CC9:-gcc} $CC9FLAGS"
 	userpath=true
-	extralibs="$extralibs -lutil -lresolv -lpthread"
+	extralibs="$extralibs -lutil -lresolv -lpthread -lucontext"
 	;;
 *Darwin*x86_64*)
 	ld="${CC9:-gcc} -m64 $CC9FLAGS"