bpf: write CO-RE relocation record size only once

The CO-RE relocation record size should be written only once in the
.BTF.ext section, not once for each section with relocations.

gcc/ChangeLog:

	* config/bpf/coreout.cc (output_btfext_header): Account for
	4-byte record size in core_relo_len.
	(output_btfext_core_sections): Only write record size once.
	* config/bpf/coreout.h (btf_ext_section_header): Delete unused
	member.

gcc/testsuite/ChangeLog:

	* gcc.target/bpf/core-section-1.c: Adjust expected record size
	occurrences.