v.pref: error with exit if coroutines not supported

[lcheylus]

When building with -use-coroutines, immediate exit with error message if coroutines not supported (OS != Linux/macOS).

• Before this fix, build on OpenBSD/amd64: error message then build continues and fails.

$ ./v -use-coroutines examples/coroutines/simple_coroutines.v
coroutines only work on macos & linux for now
================== C compilation error (from tcc): ==============
cc: tcc: error: file '/home/fox/dev/vlang.git/thirdparty/photon/photonwrapper.so' not found
=================================================================
Try passing `-g` when compiling, to see a .v file:line information, that correlates more with the C error.
(Alternatively, pass `-show-c-output`, to print the full C error message).
builder error:
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .

• With this fix, immediate exit with error message:

$ ./v -use-coroutines examples/coroutines/simple_coroutines.v
coroutines only work on macOS & Linux for now

[spytheman]

That is indeed a better choice 🤔 .
Thank you @lcheylus 🙇🏻 .