BUILD FIX: memory max constant in allocator

[travisdowns]

After recent changes to memory.cc, it fails to compile as standard
libraries vary in their return value for std::bit_width. Initial C++20
behavior was to return an integer of the same signeness as the input,
but later standards specify an int return type.

This patch changes the variable w to be an int from auto so it
works everywhere.

[travisdowns]

I see, the problem is that https://cplusplus.github.io/LWG/issue3656 changed the return type from same as input to always int, so standard libs in the wild vary. So the right fix is to use int instead of auto.