If NEON_INTRINSICS=0, then run the GAS sanity check from libjpeg-turbo
2.0.x and force-enable NEON_INTRINSICS if the test fails. This fixes
the AArch32 build when using Clang 6.0 on Linux or the Clang toolchain
in the Android NDK r15*-r16*. It also prevents users from manually
disabling NEON_INTRINSICS if doing so would break the build (such as
with Xcode 5.)
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