Enable traditional TLS accesses on arm64

Handle generic DTPMOD/DTPREL relocations on arm64. Expose __tls_get_addr.

Bug: http://b/123385182
Test: manual -- compile code using arm64 gcc -mtls-dialect=trad
Change-Id: I99d2faae35d1ebffc1be8de50b4a1f5e21b14348
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 0bcf761..0cab83d 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1485,6 +1485,7 @@
     __mempcpy_chk;
     __open64_2;
     __openat64_2;
+    __tls_get_addr; # arm64
     call_once;
     cnd_broadcast;
     cnd_destroy;