Remove architecture name from names of HOST_*.mk files.

These makefiles are actually architecture-independent (as far as I can
tell, the Darwin one doesn't have anything arch-specific and there's
just one of them anyway), so let's remove the architecture name from the
names of these files to make it easier to support new host architectures.

Bug: 294584591
Change-Id: Ic4cb0bf393c53ac9593afb7c2c6a6e07fbe6079e