dwarf: use find_abstract_instance for vars and DW_AT_specification

dwarf: use find_abstract_instance for vars and DW_AT_specification

The following simple test case fails when dwz is used:

$ cat demo.C
namespace std {
  enum { _S_fixed, _S_floatfield = _S_fixed };
  struct {
    struct {};
  }
  __ioinit;
}

int main() {
  return 0;
}

$ g++ demo.C -g && cp a.out b.out && dwz -m xxx.so a.out b.out && objdump -S a.out >/dev/null
objdump: DWARF error: could not find variable specification at offset 0x3d3

As seen the reference is defined in xxx.so shared part:

$ eu-readelf -w -N a.out | grep -A3 -B3 3d3
             decl_column          (data1) 11
             sibling              (ref_udata) [   387]
 [   387]    variable             abbrev: 30
             specification        (GNU_ref_alt) [   3d3]
             location             (exprloc)
              [ 0] addr 0x404019
 [   396]    subprogram           abbrev: 32

$ eu-readelf -w -N a.out | less

...

 Compilation unit at offset 920:
 Version: 5, Abbreviation section offset: 0, Address size: 8, Offset size: 4
 Unit type: partial (3)
...
 [   3d3]      variable             abbrev: 31
               name                 (strp) "__ioinit"
               decl_file            (data1) demo.C (10)
               decl_line            (data1) 6
               decl_column          (data1) 3
               type                 (ref_udata) [   3c4]
               declaration          (flag_present) yes

With the patch the same output is emitted as before usage of dwz.

bfd/ChangeLog:

	PR 29442
	* dwarf2.c (struct varinfo): Use const char * type.
	(scan_unit_for_symbols): Call find_abstract_instance for
	DW_AT_specification for variables that can be in a different CU
	(e.g. done by dwz)