scudo: Re-order Allocator fields for improved performance. NFCI.

Move smaller and frequently-accessed fields near the beginning
of the data structure in order to improve locality and reduce
the number of instructions required to form an access to those
fields. With this change I measured a ~5% performance improvement on
BM_malloc_sql_trace_default on aarch64 Android devices (Pixel 4 and
DragonBoard 845c).

Differential Revision: https://reviews.llvm.org/D88350