runtime: attempt to fix alignment issues with debug.sbrk
runtime: attempt to fix alignment issues with debug.sbrk

The system-based allocator backend appears to ignore alignment requirements
entirely when reusing existing allocations; I suspect this has only
recently been tested on x86 systems.

This allows various tests to pass when using the debug.sbrk backend.

I am uncertain whether this is the correct fix.

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go