Move immintrin/arm_neon includes to where they are used.

On my Mac (so, immintrin), this improves compile time, both wall and cpu,
by about 16%.  To test I ran this on an SSD with files hot in their caches:

  $ env CC=/usr/bin/clang CXX=/usr/bin/clang++ ./gyp_skia && \
    ninja -C out/Release -t clean && \
    time ninja -C out/Release

  Before: 159 wall / 3367 cpu
          159 wall / 3368 cpu

  After:  137 wall / 2860 cpu
          136 wall / 2863 cpu

I also tried further refining immintrin down to emmintrin / tmmintrin / smmintrin etc.
That made no signficant difference, so I've kept immintrin for its simplicity.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2045633002
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

TBR=reed@google.com
No public API changes.

Committed: https://skia.googlesource.com/skia/+/12dfaaa53c23f3d03050bde8f64136ac1f44164a
Review-Url: https://codereview.chromium.org/2045633002
diff --git a/include/private/SkFloatingPoint.h b/include/private/SkFloatingPoint.h
index 6ed6144..a7aa50c 100644
--- a/include/private/SkFloatingPoint.h
+++ b/include/private/SkFloatingPoint.h
@@ -15,6 +15,12 @@
 #include <math.h>
 #include <float.h>
 
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE1
+    #include <xmmintrin.h>
+#elif defined(SK_ARM_HAS_NEON)
+    #include <arm_neon.h>
+#endif
+
 // For _POSIX_VERSION
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #include <unistd.h>