Precompute hash value for symbol_set_names

We can also compute the hash for the mangled name on a background
thread so make this function even faster (about a 7% speedup).

gdb/ChangeLog:

2019-10-03  Christian Biesinger  <cbiesinger@google.com>

	* minsyms.c (minimal_symbol_reader::install): Also compute the hash
	of the mangled name on the background thread.
	* symtab.c (symbol_set_names): Allow passing in the hash of the
	linkage_name.
	* symtab.h (symbol_set_names): Likewise.