[GWP-ASan] Fix PRNG to use IE TLS.

Summary:
GWP-ASan's PRNG didn't use Initial-Exec TLS. Fix that to ensure that we don't
have infinite recursion, and also that we don't allocate a DTV on Android when
GWP-ASan is touched.

Test coverage ensuring that the sample counter is UINT32_MAX for an
uninitialised GWP-ASan is provided by gwp_asan/tests/late_init.cpp.

Reviewers: pcc, cferris

Reviewed By: pcc

Subscribers: #sanitizers, llvm-commits, rprichard, eugenis

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74135