Add sig2str()/str2sig() from POSIX Issue 8.

(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:55c1e379790545fff6239aefcb1c51d0a6463fbf)

Merged-In: I3b2c1601bd667af6b8f736e03a6e94035b812ff7
Change-Id: I3b2c1601bd667af6b8f736e03a6e94035b812ff7
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 35e67c8..ffd5d1e 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -953,6 +953,7 @@
     setvbuf;
     setxattr;
     shutdown;
+    sig2str; # introduced=36
     sigaction;
     sigaddset;
     sigaltstack;
@@ -995,6 +996,7 @@
     stdout; # var introduced=23
     stpcpy;
     stpncpy;
+    str2sig; # introduced=36
     strcasecmp;
     strcasecmp_l; # introduced=23
     strcasestr;