add control flow integrity (cfi)

[tarakby]

Issue to be solved

Add cfi in the C flags to protect the package against non intended code execution flows in the package.

CFI only available on clang and linux, and can be added by extending CFLAGS to include -fvisibility=hidden -flto -fsanitize=cfi.
The feature should be added only when building with clang on linux. Enforcing CFI for all usage of the package forces users to use clang which may add friction to importing the package.

original issue is onflow/flow-go#3668.

Suggested Solution

No response

[tarakby]

draft PR to enable CFI: #14