build: enable parallel builds in msbuild using MTT

[jeffbolznv]

msbuild performance has been poor recently, particularly since the model implementations were split into separate source files. This change enables MultiToolTask which enables parallelism within a project without the problems of /MP. See https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/.

I tested each of the vulkan and cuda backends. My CPU is a 32-core threadripper.

vulkan before
========== Rebuild started at 2:50 PM and took 03:38.550 minutes ==========
vulkan after
========== Rebuild started at 3:36 PM and took 01:57.272 minutes ==========

cuda before
========== Rebuild started at 3:27 PM and took 07:59.340 minutes ==========
cuda after
========== Rebuild started at 3:38 PM and took 06:17.516 minutes ==========

This is supported in vs2019+. I think unknown options are ignored, so this shouldn't be harmful on older versions.

[ggerganov]

I don't have an environment to test this. Feel free to merge if it works on your end.

[Acly]

Maybe guard it with if(LLAMA_STANDALONE) ?