Update norls_allocator to conform to the JIT CC.

Update norls_allocator to conform to the JIT CC.

There are no functional changes aside from the removal of a couple
of fields from norls_allocator and norls_pagdesc. As part of this
change, norls_allocator has been renamed to ArenaAllocator to better
reflect its behavior.

Commit migrated from https://github.com/dotnet/coreclr/commit/62ce0e505571896f3a63023c0074c79be1c26a60