[dwarf-reader] Better use of linkage name for fn decl de-duplication

[dwarf-reader] Better use of linkage name for fn decl de-duplication

When looking at a C program, during function decl DIE de-duplication
at we can rely on linkage names of function declarations to quickly
determine if two function decls are equal, in a given binary.

This patch uses that observation to speed up function decl DIE
de-duplication.  abidw --noout vmlinux goes from 8 to 5 minutes with
this.

	* src/abg-dwarf-reader.cc (read_context::{die_is_in_c,
	die_is_in_c_or_cplusplus}): Define new member functions.
	(fn_die_equal_by_linkage_name): Define new static function.
	(compare_dies): In the case for for DW_TAG_subprogram, use the new
	fn_die_equal_by_linkage_name.
	* tests/data/test-annotate/test15-pr18892.so.abi: Adjust.
	* tests/data/test-annotate/test21-pr19092.so.abi: Adjust.
	* tests/data/test-read-dwarf/test15-pr18892.so.abi: Adjust.
	* tests/data/test-read-dwarf/test21-pr19092.so.abi: Adjust.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>