hosted/pcm/alsa: Use alternate signal stack for the async callback.

Signals are by default executed on the user stack, i.e. the stack of
the currently active thread. This has two problems:
1) The stack size of the current stack is likely insufficient (unless
using sigaltstack threads) because our stack sizes are normally
below MINSIGSTKSIZE which is needed to deliver a signal.
2) Some of our asm code does nasty tricks with the stack pointer. When a
signal comes in during this bad things can happen, e.g. random memory
being overwritten or simply a crash.

Using a well defined stack fixes this. This is comparable with the
separate irq stack on native targets.

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