btf: improve btf-datasec-3.c test [PR114642]

btf: improve btf-datasec-3.c test [PR114642]

This test failed on powerpc --target_board=unix'{-m32}' because two
variables were not placed in sections where the test silently (and
incorrectly) assumed they would be.

The important thing for the test is only that BTF_KIND_DATASEC entries
are NOT generated for the extern variable declarations without an
explicit section attribute.  Make the test more robust by placing the
non-extern variables in explicit sections, and invert the checks to
more accurately verify what we care about in this test.

gcc/testsuite/
	PR testsuite/114642
	* gcc.dg/debug/btf/btf-datasec-3.c: Make test more robust on different
	architectures.