Add missing call to `__sanitizer::InitializePlatformEarly()` in UBSan's standalone init.

Summary:
While working on rdar://problem/62083617 I noticed this call was
missing.

This is a no-op for all platforms except Darwin. For Darwin this
means the `use_xnu_fast_mmap` flag is initialized as it was intended
when using UBSan in standalone mode.

Reviewers: vitalybuka, vsk, kubamracek, yln, samsonov

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D78532