scudo: Shrink secondary header and cache entry size by a word on Linux. NFCI.

Normally compilers will allocate space for struct fields even if the
field is an empty struct. Use the [[no_unique_address]] attribute to
suppress that behavior. This attribute that was introduced in C++20,
but compilers that do not support [[no_unique_address]] will ignore
it since it uses C++11 attribute syntax.

Differential Revision: https://reviews.llvm.org/D92966