btf: fix bootstrap -Wformat errors [PR110073]

btf: fix bootstrap -Wformat errors [PR110073]

Commit 7aae58b04b9 "btf: improve -dA comments for testsuite" broke
bootstrap on a number of architectures because it introduced some
new -Wformat errors.

Fix those errors by properly using PRIu64 and a small refactor to
the offending code.

Based on the suggested patch from Rainer Orth.

	PR debug/110073

gcc/ChangeLog:

	* btfout.cc (btf_absolute_func_id): New function.
	(btf_asm_func_type): Call it here.  Change index parameter from
	size_t to ctf_id_t.  Use PRIu64 formatter.