Use the correct HOST_ARCH for oat files.

This typo explains why test-art-host-oat-optimizing-SignalTest32
would work on a 32bit host build and fail on a 64bit host build:
because dex2oat does not generate the code in the expected path
(x86_64 instead of x86), dalvikvm would re-generate it, with
the default settings (that is with quick compiler), and hence the
test would not fail.

Change-Id: I08b5ec316deb4d669fe76066d33a91fc3ed9e3f9