[gdb] Fix build breaker with gcc 4.8

[gdb] Fix build breaker with gcc 4.8

When compiling with gcc 4.8, we run into:
...
/usr/include/c++/4.8/bits/unordered_map.h:100:18: required from \
  ‘class std::unordered_map<sect_offset, std::vector<sect_offset> >’
src/gdb/dwarf2read.h:260:5:   required from here
/usr/include/c++/4.8/bits/hashtable_policy.h:1070:12: error: invalid use of \
  incomplete type ‘struct std::hash<sect_offset>’
...

Fix this by setting the Hash template parameter of the unordered_map to
gdb::hash_enum<sect_offset>, rather than using the default
std::hash<sect_offset>.

Build and reg-tested on x86_64-linux with gcc 4.8.

gdb/ChangeLog:

2019-06-19  Tom de Vries  <tdevries@suse.de>

	* dwarf2read.h (abstract_to_concrete): Change type to
	std::unordered_map<sect_offset, std::vector<sect_offset>,
	gdb::hash_enum<sect_offset>>.