[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
Change-Id: Icd7164f96aa7115b3585c21c0f68529cd2f07c11