Convert dwarf2_cu::call_site_htab to new hash table

Convert one use of htab_t, mapping (unrelocated) pc to call_site
objects, to `gdb::unordered_map<unrelocated_addr, call_site *>`.

Change-Id: I40a0903253a8589dbdcb75d52ad4d233931f6641
Approved-By: Tom Tromey <tom@tromey.com>