[PATCH v2 2/4] DWARF 5 support: Handle DW_FORM_strx

[PATCH v2 2/4] DWARF 5 support: Handle DW_FORM_strx

* Handle DW_FORM_strx forms everywhere.

Tested with CC=/usr/bin/gcc (version 8.3.0) against master branch (also with
-gsplit-dwarf and -gdwarf-4 flags) and there was no increase in the set of
tests that fails.

This is part of an effort to support DWARF 5 in gdb.

gdb/ChangeLog:

        * dwarf2read.c (skip_one_die): Handle DW_FORM_strx forms.
        (dwarf2_string_attr): Likewise.