[scudo] Get rid of initLinkerInitialized

Now that everything is forcibly linker initialized, it feels like a
good time to get rid of the `init`/`initLinkerInitialized` split.

This allows to get rid of various `memset` construct in `init` that
gcc complains about (this fixes a Fuchsia open issue).

I added various `DCHECK`s to ensure that we would get a zero-inited
object when entering `init`, which required ensuring that
`unmapTestOnly` leaves the object in a good state (tests are currently
the only location where an allocator can be "de-initialized").

Running the tests with `--gtest_repeat=` showed no issue.

Differential Revision: https://reviews.llvm.org/D103119