[cfi] Export __cfi_shadow_size.

__cfi_shadow_size returns the size of the CFI shadow mapping, or 0 if
CFI is not (yet) used in this process. This can be used to adjust
RLIMIT_AS setting.

Test: device bionic tests
Bug: 64293803

(cherry picked from commit 97c16f8dca0694abfcdda34f8e01f5e1e989120f)

Change-Id: Ia82152f568e3a80deb902d02fa400e88e3f556bc
diff --git a/libdl/libdl.map.txt b/libdl/libdl.map.txt
index 408d4dc..d1f4ab8 100644
--- a/libdl/libdl.map.txt
+++ b/libdl/libdl.map.txt
@@ -35,6 +35,7 @@
 
 LIBC_OMR1 { # future
   global:
+    __cfi_shadow_size; # future
     __cfi_slowpath; # future
     __cfi_slowpath_diag; # future
 } LIBC_N;