btf: Emit labels in DATASEC bts_offset entries.

btf: Emit labels in DATASEC bts_offset entries.

GCC was defining bts_offset entry to always contain 0.
When comparing with clang, the same entry is instead a label to the
respective variable or function. The assembler emits relocations for
those labels.

gcc/ChangeLog:
	PR target/114431
	* btfout.cc (get_name_for_datasec_entry): Add function.
	(btf_asm_datasec_entry): Print label when possible.

gcc/testsuite/ChangeLog:
	* gcc.dg/debug/btf/btf-datasec-1.c: Correct for new
	implementation.
	* gcc.dg/debug/btf/btf-datasec-2.c: Likewise
	* gcc.dg/debug/btf/btf-pr106773.c: Likewise