Be careful about types in SkNx_neon.h.
Be careful about types in SkNx_neon.h.

The Google3 Android ARM build is still using GCC, which isn't so loosey-goosey as Clang about bit-casting back and forth between int32x4_t and uint32x4_t.

BUG=skia:

Change-Id: I0d54f6859b8e03be4936c51bbaa1967db4261bd4
Reviewed-on: https://skia-review.googlesource.com/4974
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>

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