bpf: ensure correct string offsets in BTF.ext

bpf: ensure correct string offsets in BTF.ext

BPF CO-RE relocations contain offsets to strings buffered in the BTF
string table. These BTF-specific strings are stored in memory in the
CTF auxilliary strtab, which at output time is concatenated onto the end
of the standard strtab.

Previously, these string offsets were computed at the time the
relocations were created. But strings could be added to the standard
strtab after this point, causing the offsets to no longer be correct.

Compute the offsets just before output instead, when they are sure to no
longer change.

gcc/ChangeLog:

	* config/bpf/coreout.cc (bpf_core_reloc_add): Do not account
	for base strtab offset yet as it may change.
	(output_asm_btfext_core_reloc): Do so here instead.
	(output_btfext_core_sections): Likewise.