arm: Fix building vp8_subpixelvariance_neon.c with MSVC
arm: Fix building vp8_subpixelvariance_neon.c with MSVC

Use the right return values - vget_low_s64 returns int64x1_t, not
a normal int64_t.

Also make __builtin_prefetch a no-op on MSVC for this file.

Change-Id: I4d2fce01d0ba106b98d3d53b137803119c2c2c08

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