Add execinfo functionality.

Bug: 27877410

Test: Add new unit tests.
Change-Id: Id5d7eb27a23f50e99a04f5ee1ab64047ba269bab
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 12b8114..00b25d3 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1567,6 +1567,9 @@
 
 LIBC_T { # introduced=Tiramisu
   global:
+    backtrace;
+    backtrace_symbols;
+    backtrace_symbols_fd;
     preadv2;
     preadv64v2;
     pwritev2;