update SkJumper stages to clang 5

To continue building stages, update Clang and update your GN args:
  $ brew update
  $ brew upgrade llvm
  $ find . | grep args.gn | xargs sed -ie 's/clang-4.0/clang-5.0/g'

Some interesting codegen changes I noticed:
  - ARMv7: generally better register assignment, tighter code
  - ARMv7: dropped the 128-bit alignment hint when loading and storing dst "registers",
           unclear why.
  - HSW:   now clearing the destination register before vgatherdps,
           to break a dependency on the previous value

Change-Id: I4f804a4cbfcde530fad5ed535438174e852a9593
Reviewed-on: https://skia-review.googlesource.com/44241
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/jumper/build_stages.py

@@ -9,7 +9,7 @@ import re
 import subprocess
 import sys
 
-clang         = 'clang-4.0'
+clang         = 'clang-5.0'
 objdump       = 'gobjdump'
 ccache        = 'ccache'
 stages        = 'src/jumper/SkJumper_stages.cpp'