[scudo][standalone] Work with -Werror=class-memaccess

Summary:
Fuchsia's gcc uses this, which in turn prevents us to compile successfully
due to a few `memset`'ing some non-trivial classes in some `init`.

Change those `memset` to members initialization.

Reviewers: pcc, hctim

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D77902

GitOrigin-RevId: 612f23857f36669a12ccdc1a6e93781080907203
3 files changed