Documentation/mic/mpssd: don't build x86 userspace when cross compiling

The following was seen in linux-next build coverage, which is somewhat
unique since it uses powerpc host to cross compile x86:

Documentation/mic/mpssd/mpssd.c:93:10: error: braced-group within expression allowed only inside a function
Documentation/mic/mpssd/mpssd.c:96:10: error: braced-group within expression allowed only inside a function
Documentation/mic/mpssd/mpssd.c:113:10: error: braced-group within expression allowed only inside a function
Documentation/mic/mpssd/mpssd.c:116:10: error: braced-group within expression allowed only inside a function
Documentation/mic/mpssd/mpssd.c:119:3: error: initializer element is not constant
Documentation/mic/mpssd/mpssd.c:119:3: error: (near initialization for 'virtnet_dev_page.host_features')
Documentation/mic/mpssd/mpssd.c:146:10: error: braced-group within expression allowed only inside a function
Documentation/mic/mpssd/mpssd.c:149:3: error: initializer element is not constant
Documentation/mic/mpssd/mpssd.c:149:3: error: (near initialization for 'virtblk_dev_page.host_features')
Documentation/mic/mpssd/mpssd.c:151:3: error: initializer element is not constant
Documentation/mic/mpssd/mpssd.c:151:3: error: (near initialization for 'virtblk_dev_page.blk_config.seg_max')
Documentation/mic/mpssd/mpssd.c:152:3: error: initializer element is not constant
Documentation/mic/mpssd/mpssd.c:152:3: error: (near initialization for 'virtblk_dev_page.blk_config.capacity')
make[5]: *** [Documentation/mic/mpssd/mpssd.o] Error 1

Since it is building /usr/sbin/mpssd and /usr/sbin/micctrl
for x86_64 and the original authors indicated[1] that:

   MIC card is expected to work with x86_64 host, not with ppc64.
   We have never compiled on ppc host..

so it probably makes sense to just skip building these userspace
programs when we are cross compiling.

[1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-December/123296.html

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: Sudeep Dutt <sudeep.dutt@intel.com>
Cc: Caz Yokoyama <Caz.Yokoyama@intel.com>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/mic/mpssd/Makefile

+ifndef CROSS_COMPILE
 # List of programs to build
 hostprogs-$(CONFIG_X86_64) := mpssd
 
@@ -17,3 +18,4 @@ HOSTLOADLIBES_mpssd := -lpthread
 install:
 	install mpssd /usr/sbin/mpssd
 	install micctrl /usr/sbin/micctrl
+endif