decode: fix NEON inclusion (#714)

The macro that checks for NEON support should be __ARM_NEON, not
__ARM_NEON__. [1]

AArch64 compilers define __ARM_NEON but not __ARM_NEON__.
AArch32 compilers currently seem to define both, but could be within their
rights to drop __ARM_NEON__ in future versions.

This change moves the check into the common/platform.h file, checks for
both forms, and sets BROTLI_TARGET_NEON if NEON support is available.

[1] Section 6.5.4 of the ARM C Language Extensions.
    (At the time of writing, the latest version was Release 2.1.)