BTF: fix PR debug/112768

BTF: fix PR debug/112768

PR debug/112768 - btf: fix asm comment output for BTF_KIND_FUNC* kinds

The patch adds a small function to abstract out the detail and return
the name of the type.  The patch also fixes the issue of BTF_KIND_FUNC
appearing in the comments with a 'null' string.

For btf-function-6.c testcase, after the patch:

        .long   0       # TYPE 2 BTF_KIND_FUNC_PROTO ''
        .long   0xd000002       # btt_info: kind=13, kflag=0, vlen=2
        .long   0x1     # btt_type: (BTF_KIND_INT 'int')
        .long   0       # farg_name
        .long   0x1     # farg_type: (BTF_KIND_INT 'int')
        .long   0       # farg_name
        .long   0x1     # farg_type: (BTF_KIND_INT 'int')
        .long   0       # TYPE 3 BTF_KIND_FUNC_PROTO ''
        .long   0xd000001       # btt_info: kind=13, kflag=0, vlen=1
        .long   0x1     # btt_type: (BTF_KIND_INT 'int')
        .long   0x68    # farg_name
        .long   0x1     # farg_type: (BTF_KIND_INT 'int')
        .long   0x5     # TYPE 4 BTF_KIND_FUNC 'extfunc'
        .long   0xc000002       # btt_info: kind=12, kflag=0, linkage=2
        .long   0x2     # btt_type: (BTF_KIND_FUNC_PROTO '')
        .long   0xd     # TYPE 5 BTF_KIND_FUNC 'foo'
        .long   0xc000001       # btt_info: kind=12, kflag=0, linkage=1
        .long   0x3     # btt_type: (BTF_KIND_FUNC_PROTO '')

gcc/ChangeLog:

	PR debug/112768
	* btfout.cc (get_btf_type_name): New definition.
	(btf_collect_datasec): Update dtd_name to the original type name
	string.
	(btf_asm_type_ref): Use the new get_btf_type_name function
	instead.
	(btf_asm_type): Likewise.
	(btf_asm_func_type): Likewise.

gcc/testsuite/ChangeLog:

	PR debug/112768
	* gcc.dg/debug/btf/btf-function-6.c: Empty string expected with
	BTF_KIND_FUNC_PROTO.