[compiler-rt] Simplify and rename of operator_new_size_type

We can rely on the compiler-provided macro __SIZE_TYPE__ for all
non-MSVC compilers and fall back to `uptr` otherwise.
I verified via https://godbolt.org/z/MW9KMjv5f that this works for MSVC
as well as GCC 4.5 Clang 3.0, so that should cover supported compilers.

While touching this also rename operator_new_size_type to usize which
makes it more obvious that this is the equivalent to size_t within
the sanitizers runtime (which I plan to use in follow-up changes).

Reviewed By: vitalybuka

Pull Request: https://github.com/llvm/llvm-project/pull/83912