Use libxxhash for hashing, if present

XXHash is faster than htab_hash_string:
------------------------------------------------------------
Benchmark                     Time           CPU Iterations
------------------------------------------------------------
BM_xxh3                      11 ns         11 ns   65887249
BM_xxh32                     19 ns         19 ns   36511877
BM_xxh64                     16 ns         16 ns   42964585
BM_hash_string              182 ns        182 ns    3853125
BM_iterative_hash            77 ns         77 ns    9087638

Unfortunately, XXH3 is still experimental (see
https://github.com/Cyan4973/xxHash#user-content-new-experimental-hash-algorithm)

However, regular XXH64 is still a lot faster than
htab_hash_string per my benchmark above. I used the
following string for the benchmark:
static constexpr char str[] = "_ZZZL13make_gdb_typeP7gdbarchP10tdesc_typeEN16gdb_type_creator19make_gdb_type_flagsEPK22tdesc_type_with_fieldsE19__PRETTY_FUNCTION__";

htab_hash_string is currently 4.35% + 7.98% (rehashing) of gdb
startup when attaching to Chrome's content_shell.

An additional 5.21% is spent in msymbol_hash, which does not use
this hash function. Unfortunately, since it has to lowercase the
string, it can't use this hash function.
BM_msymbol_hash                52 ns         52 ns   13281495

It may be worth investigating if strlen+XXHash is still faster than
htab_hash_string, which would make it easier to use in more places.

Debian ships xxhash as libxxhash{0,-dev}. Fedora ships it as xxhash-devel.

gdb/ChangeLog:

2019-10-22  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in: Link with libxxhash.
	* config.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Search for libxxhash.
	* utils.c (fast_hash): Use xxhash if present.

Change-Id: Icab218388b9f829522ed3977f04301ae6d4fc4ca

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