fix(wasi): update wasi stubs

[ricochet]

Today, WASI does not support atomics. Support for WASI in Abseil was added in #1509 and needs to be refreshed.

Note that there is a roadmap item for wasi that will give us access to atomics via the component model's cooperative threads feature. Implementations are in progress but this may take a few months and so in the mean time, we need this fix.

I created an example with the necessary stubs that are missing from upstream abseil and validated that these can be removed with the changes in this PR: https://github.com/ricochet/wasi-protobuf/blob/main/wasi-build/wasi_stubs.cpp

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[derekmauro]

It should be far simpler to achieve this. I believe all that needs to be done is a slight tweak here and everything else should just work:

abseil-cpp/absl/base/internal/low_level_alloc.h

Lines 34 to 41 in 9d35bf5

	// LowLevelAlloc requires that the platform support low-level
	// allocation of virtual memory. Platforms lacking this cannot use
	// LowLevelAlloc.
	#ifdef ABSL_LOW_LEVEL_ALLOC_MISSING
	#error ABSL_LOW_LEVEL_ALLOC_MISSING cannot be directly set
	#elif !defined(ABSL_HAVE_MMAP) && !defined(_WIN32)
	#define ABSL_LOW_LEVEL_ALLOC_MISSING 1
	#endif

[ricochet]

Thank you that simplified it quite a bit 👍