[Sanitizer][Darwin] Cleanup MaybeReexec() function and usage

[Sanitizer][Darwin] Cleanup MaybeReexec() function and usage

While investigating another issue, I noticed that `MaybeReexec()` never
actually "re-executes via `execv()`" anymore.  `DyldNeedsEnvVariable()`
only returned true on macOS 10.10 and below.

Usually, I try to avoid "unnecessary" cleanups (it's hard to be certain
that there truly is no fallout), but I decided to do this one because:

* I initially tricked myself into thinking that `MaybeReexec()` was
  relevant to my original investigation (instead of being dead code).
* The deleted code itself is quite complicated.
* Over time a few other things were mushed into `MaybeReexec()`:
  initializing `MonotonicNanoTime()`, verifying interceptors are
  working, and stripping the `DYLD_INSERT_LIBRARIES` env var to avoid
  problems when forking.
* This platform-specific thing leaked into `sanitizer_common.h`.
* The `ReexecDisabled()` config nob relies on the "strong overrides weak
  pattern", which is now problematic and can be completely removed.
* `ReexecDisabled()` actually hid another issue with interceptors not
  working in unit tests.  I added an explicit `verify_interceptors`
  (defaults to `true`) option instead.

Differential Revision: https://reviews.llvm.org/D129157