ANDROID: modpost: add an exception for CFI stubs

When CONFIG_CFI_CLANG is enabled, LLVM renames all address taken
functions by appending a .cfi postfix to their names, and creates
function stubs with the original names. The compiler always injects
these stubs to the text section, even if the function itself is
placed into init or exit sections, which creates modpost warnings.
This commit adds a modpost exception for CFI stubs to prevent the
warnings.

Bug: 117237524
Change-Id: Ieb8bf20d0c3ad7b7295c535f598370220598cdb0
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>