Emit i386 path in arm_neon.h

To include arm_neon_x86.h renamed from
aosp/gcc/gcc-4.x/gcc/config/i386/arm_neon.h by
ndk/build/tools/build-llvm.sh for __i386__

See https://android-review.googlesource.com/#/c/107511 and
https://software.intel.com/en-us/blogs/2012/12/12/from-arm-neon-to-intel-mmxsse-automatic-porting-solution-tips-and-tricks

Change-Id: I9273e4d2ed9b14a9e3bad5dc62961feb838a716a