We already have a LICENSE file that covers the Zig Standard Library. We
no longer need to remind everyone that the license is MIT in every single
file.
Previously this was introduced to clarify the situation for a fork of
Zig that made Zig's LICENSE file harder to find, and replaced it with
their own license that required annual payments to their company.
However that fork now appears to be dead. So there is no need to
reinforce the copyright notice in every single file.
Let's a void any kind of compilation/LLVM errors for niche targets such
as AVR/MSP430 or ARM v6m. By not exporting any atomic builtin anymore
the user is free to provide their own implementation (that disable the
IRQs) or to provide the --single-threaded switch and forget about this.
The v6m ISA has no way to express a CAS loop natively without turning
off the interrupts or using the kernel cmpxchg harness.
On such a platform the user has to provide a few __sync_* builtins to
satisfy the linker.
