[scudo] Make Secondary linker-initialized compliant

Summary:
As a follow up to D48142 for Scudo, switch the `SpinMutex` to its static
counterpart, and ensure zero-initialization by memset'ing the whole class.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D48148

llvm-svn: 334716