xcc: do not call ASSERT in CODE_UNREACHABLE

230ddd9a added unreachable path hint for assertion failure. Xcc does
not support __builtin_unreachable and this was defined as a call to
ASSERT, which generated some recursion.

Replace the definition with __builtin_trap() to work around this issue.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>