[sanitizer] Use __atomic_load/store() built-ins for generic 32-bit targets

Simplifies the code and fixes the build on SPARC.
See discussion in: http://lists.llvm.org/pipermail/llvm-dev/2020-October/145937.html

Author: glaubitz (John Paul Adrian Glaubitz)
Reviewed-in: https://reviews.llvm.org/D89940