[sanitizer] Reduce redzone size for small size global objects

Currently 1 byte global object has a ridiculous 63 bytes redzone.
This patch reduces the redzone size to be less than 32 if the size of global object is less than or equal to half of 32 (the minimal size of redzone).
A 12 bytes object has a 20 bytes redzone, a 20 bytes object has a 44 bytes redzone.

Reviewed By: MaskRay, #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D102469