mach_vm_read_overwrite instead of mach_vm_read

Hi,

This isn’t something that is very important, but I was looking through the code and noticed that lldb uses mach_vm_read() and then a memcpy() (and mach_vm_deallocate()). It should, I think, be more efficient (and slightly simpler) to use mach_vm_read_overwrite() instead, the following should have the same net result with less code:

Index: tools/debugserver/source/MacOSX/MachVMMemory.cpp