scudo: Add an API for disabling memory initialization per-thread.

Here "memory initialization" refers to zero- or pattern-init on
non-MTE hardware, or (where possible to avoid) memory tagging on MTE
hardware. With shared TSD the per-thread memory initialization state
is stored in bit 0 of the TLS slot, similar to PointerIntPair in LLVM.

Differential Revision: https://reviews.llvm.org/D87739