Blender Git Loki

Cycles: Support WITH_CYCLES_NATIVE_ONLY with MSVC

This change enables the developer option `WITH_CYCLES_NATIVE_ONLY`
for MSVC. This allows a developer to just build the cycles
CPU kernel for their specific system rather than all kernels,
speeding up development.

Other platforms have had this option for years, but MSVC lacks
the compiler switch to target the host architecture hence it
always build all kernels.

This change uses a small helper program to detect the required
flags.

Only AVX/AVX2 are tested, for the following reasons

- SSE2 is enabled by default and requires no flags
- SSE3/4 have no specific build flags for msvc
- AVX512 is not yet supported by cycles

Differential Revision: https://developer.blender.org/D8775

Reviewed by: brecht, sergey