bpf: Corrected index computation when present with unnamed struct fields

bpf: Corrected index computation when present with unnamed struct fields

Any unnamed non-struct-or-union field is not a member of the
BTF_KIND_STRUCT.
For that reason, CO-RE access strings indexes should take that in
consideration. This patch adds a condition to the incrementer that
computes the index for the field access.

gcc/ChangeLog:
	* config/bpf/core-builtins.cc (bpf_core_get_index): Check if
	field contains a DECL_NAME.

gcc/testsuite/ChangeLog:
	* gcc.target/bpf/core-builtin-fieldinfo-offset-1.c: Add
	testcase for unnamed fields.