Fix checking for Wasm SIMD support

The defined(WEBP_ENABLE_WASM_SIMD) in dsp.h meant that cpu.cmake will
never successfully compile with Wasm SIMD, since it doesn't set any
compile flags. We remove that requirement, and instead determine in
dec.c if we want to be using Wasm SIMD intrinsics.

Bug: v8:12371
Change-Id: I5d313f6d95301e6153254229a9ea234fec990cd6