arc: support microkernel on ARC

Microkernel on ARC works fine, was missing some declarations in the
linker file.
Also enable testing of microkernel with ARC and disable tests where
ARC is not supported yet.

Jira: ZEP-396
Change-Id: I2ac7b8dc0bea22f5d2e24832d9e3afad8df9f580
Signed-off-by: Anas Nashif <anas.nashif@intel.com>