Truncate address in amd64_nacl_software_single_step. If breakpoint is set to untruncated address, i…

Truncate address in amd64_nacl_software_single_step.

If breakpoint is set to untruncated address, it is not recognized by
gdb as its own breakpoint. When the breakpoint is hit, rip is not decreased
and so execution starts from the middle of the instruction.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2947
TEST= https://chromiumcodereview.appspot.com/10826183/

R=khim@chromium.com,eaeltsin@google.com

Committed: https://git.chromium.org/gitweb?p=native_client/nacl-gdb.git;a=commit;h=1e4c87f

[khim, 2012-08-09 12:13:47]

LGTM