UEFI: add support for tasks scheduler and make it default

[sparques]

This adds support to uefi-amd64 target for the tasks scheduler and makes it default. The none-scheduler is still supported.

I did a very basic test of making sure two separate goroutines ran simultaneously--seems to work.

I'm not sure if the bit in compileopts/config.go is kosher; Is that the right approach or is there a better way to conditionally include ExtraFiles?

[deadprogram]

You mean like this? https://github.com/tinygo-org/tinygo/blob/dev/targets/avr.json#L20

[sparques]

I'm sorry, if there is something that's making src/internal/task/task_stack_avr.S conditionally included, I'm missing it. Does it not matter if an assembly file is included but not used?

[deadprogram]

I am wonder why you would need it conditionally included when you can just add to target file?