This allows stack overflows to print stack traces. The size of the
sigaltstack (and whether it is actually set) can be configured by
setting `std.Options.signal_stack_size`.

The default value for the signal stack size was chosen experimentally by
doubling the value required to get stack traces on stack overflow with
the self-hosted x86_64 backend. While some targets may typically use
more stack space than x86_64-linux, the self-hosted x86_64 backend is
quite wasteful with stack at the moment, making it a fair benchmark.
Executables produced by the LLVM backend should have lower stack usage.