poco: not available for all architectures

Missing defines for:
aarch64, arc, blackfin, microblaze & xtensa.
Not properly supported: mips - only defines generic mips as BE, doesn't
know about mipsel (LE) thus assuming it's BE.
Fixes:
http://autobuild.buildroot.net/results/9847702b046bed59b07f0e075a58b1f31e9236ce/



This should be pretty straightforward to fix in
Foundation/include/Poco/Platform.h for interested parties since it only
cares about endianness.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>