Windows: Platform specific MIDL output

Add $(CPU) to the MIDL generated _c.c and _s.c output files
to prevent conflicts.

Change-Id: Ie10c85fa0b9cab19b107b44c2cf452925b8ee521
Reviewed-on: http://gerrit.openafs.org/10907
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>