[sanitizer] Minor refactor of ThreadDescriptorSize

Summary:
While I was sifting through dead code findings, I stumbled on this function.

First, for `__s390__` it always returned 0 for the 1st call, which doesn't seem
right. 2nd call & beyond would return the correct value though.
Then it duplicated the `atomic_store` multiple times, sometimes with a `if`,
sometimes without. Finally it used a capitalized variable name starting with `k`
which indicates a constant, and it is not.

So:
- rename the static global variable;
- change the atomic functions to their relaxed version;
- move the store to the end, and make sure we return `val` all the time.

Reviewers: alekseyshl, eugenis, koriakin

Reviewed By: alekseyshl

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D45725

llvm-svn: 330268