libctf, tests: do not assume host and target have identical field offsets

libctf, tests: do not assume host and target have identical field offsets

The newly-introduced libctf-lookup unnamed-field-info test checks
C compiler-observed field offsets against libctf-computed ones
by #including the testcase in the lookup runner as well as
generating CTF for it.  This only works if the host, on which
the lookup runner is compiled and executed, is the same architecture as
the target, for which the CTF is generated: when crossing, the trick
may fail.

So pass down an indication of whether this is a cross into the
testsuite, and add a new no_cross flag to .lk files that is used to
suppress test execution when a cross-compiler is being tested.

libctf/
	* Makefile.am (check_DEJAGNU): Pass down TEST_CROSS.
	* Makefile.in: Regenerated.
	* testsuite/lib/ctf-lib.exp (run_lookup_test): Use it to
	implement the new no_cross option.
	* testsuite/libctf-lookup/unnamed-field-info.lk: Mark as
	no_cross.