Add reallocarray(3).

Originally a BSD extension, now in glibc too. We've used it internally
for a while.

(cherry-pick of e4b13f7e3ca68edfcc5faedc5e7d4e13c4e8edb9.)

Bug: http://b/112163459
Test: ran tests
Change-Id: I813c3a62b13ddb91ba41e32a5a853d09207ea6bc
Merged-In: I813c3a62b13ddb91ba41e32a5a853d09207ea6bc
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index 3e8cea2..55fd587 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -1354,6 +1354,7 @@
     android_fdsan_set_error_level;
     android_get_device_api_level;
     pthread_sigqueue;
+    reallocarray;
     timespec_get;
 } LIBC_P;