[scudo] Add SCUDO_ENABLE_HOOKS to enable hooks at compilation time

Accessing the PLT entries of hooks can lead a certain amount of
performance overhead. This is observed on certain tasks which will do a
bunch of malloc/free and their throughputs are impacted by the null
check of hooks.

Also add SCUDO_ENABLE_HOOKS_TESTS to select if we want to run the hook
tests. On some platforms they may have different ways to run the
wrappers tests (end-to-end tests) and test the hooks along with the
wrappers tests may not be feasible. Provide an option to turn it ON/OFF.

By default, we only verify the hook behavior in the scudo standalone
tests if SCUDO_ENABLE_HOOKS is defined or COMPILER_RT_DEBUG is true.

Reviewed By: cferris, fabio-d

Differential Revision: https://reviews.llvm.org/D158784