[phobos] std.parallelism's unit tests randomly hang on win32

[David Simcha]

Update:  The segfaults on Linux64 are also being caused by the low order 
bit corruption bug.  Whenever I look at the chain of pointer 
dereferences in GDB by viewing the registers and disassembly on crash, 
the segfault is always caused by dereferencing a pointer to some memory 
address that's clearly illegal.  (On x64 user mode addresses can't have 
their high order bits set, and the addresses being dereferenced often 
do.  See http://en.wikipedia.org/wiki/X64#Virtual_address_space_details 
.)  This wild pointer is obtained by dereferencing another pointer whose 
low order bits are always equal to TaskStatus.done.  For example, if 
TaskStatus.done == 2, the pointer might be something like 0x0000ABCD 
EF123402.  If TaskStatus.done == 1, it will be something like 0x0000ABCD 
EF123401.

Therefore, somehow the low order bits of pointers are getting corrupted 
with the value of TaskStatus.done in several places.  This is strong 
evidence that the underlying issue is a codegen bug or a bug in the ASM 
for the atomic ops, not a concurrency bug.