`HAVE_GCC_INLINE_ASSEMBLY` Not Set Correctly on ARM64

[insertinterestingnamehere]

Noticed this while fixing #224. Something's off on our detection logic for the HAVE_GCC_INLINE_ASSEMBLY macro on ARM64. That said, I'm mainly seeing this in older sections of code that we likely want to just refactor to depend on C11 atomics instead of maintaining our own implementations, so it may be better to just eliminate that check entirely when fixing #225. Documenting this now so we can revisit that decision later.