[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 97c16f8d)

Change-Id: Ia82152f568e3a80deb902d02fa400e88e3f556bc