[scudo] Align objects with alignas

Operator new must align allocations for types with large alignment.

Before c++17 behavior was implementation defined and both clang and gc++
before 11 ignored alignment. Miss-aligned objects mysteriously crashed
tests on Ubuntu 14.

Alternatives are compile with -std=c++17 or -faligned-new, but they were
discarded as less portable.

Reviewed By: hctim

Differential Revision: https://reviews.llvm.org/D101874