tools: hv: Fix cross-compilation

tools: hv: Fix cross-compilation

Use the native ARCH only in case it is not set, this will allow the
cross-compilation where ARCH is explicitly set.

Additionally, simplify the ARCH check to build the fcopy daemon only
for x86 and x86_64.

Fixes: 82b0945ce2c2 ("tools: hv: Add new fcopy application based on uio driver")
Reported-by: Adrian Vladu <avladu@cloudbasesolutions.com>
Closes: https://lore.kernel.org/linux-hyperv/PR3PR09MB54119DB2FD76977C62D8DD6AB04D2@PR3PR09MB5411.eurprd09.prod.outlook.com/
Co-developed-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Signed-off-by: Aditya Garg <gargaditya@linux.microsoft.com>
Reviewed-by: Roman Kisel <romank@linux.microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>