vbutil: accept amd64 as a valid alias for x86
vbutil: accept amd64 as a valid alias for x86

The rest of the chromiumos build system uses amd64 as the
architecture name for 64bit x86.  This adds support for this
name to vbutil.

BUG=chromium-os:21284
TEST=vbutil --arch amd64 should not return unknown architecture

Change-Id: I37531591a7a31486f6447ae611d54569d1ea59d5
Reviewed-on: http://gerrit.chromium.org/gerrit/9959
Tested-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>

marineam/seismograph,acorn-marvell/vboot_reference,coreboot/vboot,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,ccaapton/vboot_reference,ccaapton/vboot_reference,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,ionave/seismograph,ionave/seismograph,ccaapton/vboot_reference,marineam/seismograph,ionave/seismograph,coreboot/vboot,coreboot/vboot,marineam/seismograph,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,ccaapton/vboot_reference,acorn-marvell/vboot_reference,coreboot/vboot,marineam/seismograph