gdbsupport: add gdb::string_view_hash

Add the string_view_hash type, which will be useful to be able to use
gdb::string_view as std::unordered_map keys.

Use it in gdb/symtab.c, to exercise it.

Change-Id: Id69a466ab19a9f6620b5df8a2dd29b5cddd94c00
Approved-By: Andrew Burgess <aburgess@redhat.com>