Introduce dw2_linkage_name.

Introduce dw2_linkage_name.

The DWARF reader is littered with the following idiom to read a linkage name
from the debug info:

  mangled = dwarf2_string_attr (die, DW_AT_linkage_name, cu);
  if (mangled == NULL)
    mangled = dwarf2_string_attr (die, DW_AT_MIPS_linkage_name, cu);

This patch introduces a new function to simplify this to:

  mangled = dw2_linkage_name (die, cu);

gdb/ChangeLog