make : add support for building on DragonFlyBSD/NetBSD/OpenBSD

[przemoc]

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[przemoc]

*BSDs do not have /proc/cpuinfo equivalent, so users of older machines have to remove or comment out locally in Makefile:

ifeq ($(UNAME_M),amd64)
    CFLAGS += -mavx -mavx2 -mfma -mf16c
endif

I plan to attempt addressing it in the future by writing small single source file cpuinfo app, that could be built early in Makefile for *BSDs and be used to guide Makefile what compiler flags can be used to utilize all available instruction sets instead of forcing all of them blindly.

[przemoc]

But the plan I mentioned in my previous comment is for the future (other weekend) and different PR.
There is nothing preventing this PR to be merged as-is, as it's not making anything worse than currently is.

Example from running:
./main -m models/ggml-tiny.en.bin -f samples/jfk.wav
on 4 *BSD in VMs (whisper.cpp built with local Makefile modification as described in my previous comment)