Improve reability of GC on win64

[Temtaime via Digitalmars-d]

Hi !
I have an app with large amount of math, so there's lot of arrays 
with floats.
I found that sometimes my app starts to eat memory and then it 
crash.

The problem i think is false pointers. For example i have a 
struct with pointers and static array of floats. GC marks entire 
struct as containing pointers. And when some data in the array 
starts to point to valid memory region, gc won't release that 
memory.

Also i build my app as 64 bit. I found that gc allocates memory 
using
https://github.com/D-Programming-Language/druntime/blob/1f957372e5dadb92ab1d621d68232dbf8a2dbccf/src/gc/os.d#L64

And all the addresses if i print them are below 4G. So only low 
32 bits of 64 bit address are used.

One can give preferred address to VirtualAlloc and make it > 4G.
I changed implementation of os_mem_map to something like:

ulong addr = 1 << 40;

while(true) {
if(auto p = VirtualAlloc(cast(void *)addr, nbytes, MEM_RESERVE | 
MEM_COMMIT,
                 PAGE_READWRITE)) return p;
addr += nbytes;
}

And my problem with growing memory had gone.
I think we should alter implementation of os_mem_map to allocate 
using addresses > 4G, so high bits won't be zeros and it'll help 
with false pointer.

I think we should use some random function here. Maybe rdtsc ? Or 
maybe there's other opinions ?

Thanks.