The default heap size on Windows, both 32 and 64-bit, is 1400mb. This makes
sense on 32-bit Windows due to the address space break at 2gb, but doesn't
on 64-bit Windows. We should remove the restriction for 64-bit platforms.
There's #ifdef'ed AMD64 code in src/os_cpu/linux_x86/vm/os_linux_x86.cpp that should
probably by #ifdef'ed _LP64. If it were, we could move allocatable_physical_memory
back to os_linux.cpp.