Don't emit address size prefixes for native x86-64 ABI.

Address size prefixes are used in 64-bit x86 for PNaCl's use of the
x32 ABI with ILP32 data model. Don't emit them for any other ABI.

BUG=swiftshader:9

Change-Id: I1351db086d44ce4b144b3428866a54e84637b9a4
Reviewed-on: https://chromium-review.googlesource.com/390409
Reviewed-by: Jim Stichnoth <stichnot@chromium.org>
Tested-by: Nicolas Capens <nicolascapens@google.com>