Add POSIX qsort_r().

POSIX added this to issue 8, based on https://www.austingroupbugs.net/view.php?id=900.

This is an incompatibility with iOS, which is one reason why I hadn't
added this previously, but FreeBSD (upstream for both Android and iOS in
this case) already moved off their different qsort_r(), so it's possible
Apple will, and in the meantime, at least I now have the defense of
"POSIX says...".

Change-Id: Iab048a3a931010c3276a88d25eb56b38d4d608b1
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index cc2701b..0a4db45 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -844,6 +844,7 @@
     pwrite;
     pwrite64;
     qsort;
+    qsort_r; # introduced=36
     quick_exit;
     raise;
     rand;