Use xxhash for fast --build-id.
The speed improvements I got were:
firefox
master 7.
050784981
patch 6.
842361079 0.
970439617353
chromium
master 4.
260626249
patch 4.
183148025 0.
981815296749
chromium fast
master 1.
829028591
patch 1.
806439277 0.
987649556649
the gold plugin
master 0.
336154128
patch 0.
331893374 0.
987324998728
clang
master 0.
561869781
patch 0.
558640828 0.
994253200458
llvm-as
master 0.
034025959
patch 0.
033984389 0.
99877828572
the gold plugin fsds
master 0.
360710529
patch 0.
356483564 0.
988281559145
clang fsds
master 0.
640518422
patch 0.
632329874 0.
987215749432
llvm-as fsds
master 0.
031569416
patch 0.
030822055 0.
976326423017
scylla
master 3.
154770529
patch 3.
11982016 0.
988921422754
llvm-svn: 282505