[scudo] Allow for weak hooks, gated by a define

Summary:
Hooks in the allocation & deallocation paths can be a security risk (see for an
example https://scarybeastsecurity.blogspot.com/2016/11/0day-exploit-advancing-exploitation.html
which used the glibc's __free_hook to complete exploitation).

But some users have expressed a need for them, even if only for tests and
memory benchmarks. So allow for `__sanitizer_malloc_hook` &
`__sanitizer_free_hook` to be called if defined, and gate them behind a global
define `SCUDO_CAN_USE_HOOKS` defaulting to 0.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42430

llvm-svn: 323278