DWARF: Emit address of __gshared variable correctly on I64

Picked up when testing gdb development with D patches.

When writing out ``DW_AT_location`` of a 64bit ``DW_OP_addr``, DMD only pads out the value to 32bits.

eg: ``<8a>   DW_AT_location    : 5 byte block: 3 0 0 0 0  (DW_OP_addr: 0)``

This could cause a debugger to read the variable data incorrectly, so best writing out the location with the correct length.  ``<8a>   DW_AT_location    : 9 byte block: 3 0 0 0 0 0 0 0 0  (DW_OP_addr: 0)``