Fix alignment problems in NEON Sk4b.

Fix alignment problems in NEON Sk4b.

As written at head, the compiler can assume these loads and stores are 4
byte aligned [1].  We want Sk4b to load from any 1-byte aligned address,
to prevent crashes like [2].

[1] https://llvm.org/bugs/show_bug.cgi?id=24421
[2] https://luci-milo.appspot.com/swarming/task/304079e125b1b910/steps/nanobench/0/stdout

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2183133002
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review-Url: https://codereview.chromium.org/2183133002