We need to have seg_mapping_table aligned on its natural alignement (ptr size). When...

We need to have seg_mapping_table aligned on its natural alignement (ptr size). When it's not aligned,
aside from the perf problem, we also have a functional problem when an address that's not on a heap
segment is passed - we could read an intermediate value which would cause an AV. If the address is
on a heap segment it means we are guaranteed to read a ptr size atomically. This only affects functions
like IsHeapPointer which can be run when EE is not suspended.

Commit migrated from https://github.com/dotnet/coreclr/commit/6a4e02a4ded34e385b7249ed5972b49ad97f354a