Memory releated GC options can now be specified with higher accuracy

Memory-related GC options stored in bytes instead of megabytes to
provide fine-tune GC on low memory devices.

Added ability to use suffixes B, K, M or G:
---
extern(C) __gshared string[] rt_options =
    [ "gcopt=minPoolSize:4K maxPoolSize:2M incPoolSize:8K" ];
---

Values without suffix treated as megabytes.
