libc: hide alphasort_thunk behind I_AM_SCANDIR_B

Should unbreak gcc build as reported by tinderbox:
lib/libc/gen/scandir.c:59:12: warning: 'alphasort_thunk' declared 'static' but never defined [-Wunused-function]

(cherry picked from commit 992bcb37c28ad1a6426df7996f297187ae3d901a)