sanitizer_common: bump default file max size to 256MB

I am hitting some cases where /proc/self/maps does not fit into 64MB.
256MB is lots of memory, but it's not radically more than the current 64MB.
Ideally we should read/parse these huge files incrementally,
but that's lots of work for a debugging/introspection interface.
So for now just bump the limit.

Depends on D112793.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D112794