Fix build error on aarch64

[DazWorrall]

Ref: #6311

Building on aarch64 currently fails with:

ninja: job failed: /usr/bin/c++  -I/root/binaryen/src -I/root/binaryen/third_party/llvm-project/include -I/root/binaryen -static -DBUILD_LLVM_DWARF -Wall -Werror -Wextra -Wno-unused-parameter -Wno-dangling-pointer -fno-omit-frame-pointer -fno-rtti -Wno-implicit-int-float-conversion -Wno-unknown-warning-option -Wswitch-Wimplicit-fallthrough -Wnon-virtual-dtor -fPIC -fdiagnostics-color=always -O3 -DNDEBUG -UNDEBUG -std=c++17 -MD -MT src/passes/CMakeFiles/passes.dir/Precompute.cpp.o -MF src/passes/CMakeFiles/passes.dir/Precompute.cpp.o.d -o src/passes/CMakeFiles/passes.dir/Precompute.cpp.o -c /root/binaryen/src/passes/Precompute.cpp
In file included from /root/binaryen/src/wasm-traversal.h:30,
                 from /root/binaryen/src/pass.h:24,
                 from /root/binaryen/src/ir/intrinsics.h:20,
                 from /root/binaryen/src/ir/effects.h:20,
                 from /root/binaryen/src/passes/Precompute.cpp:30:
In copy constructor 'wasm::SmallVector<wasm::Expression*, 10>::SmallVector(const wasm::SmallVector<wasm::Expression*, 10>&)',
    inlined from 'constexpr std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _U1 = wasm::Select* const; _U2 = wasm::SmallVector<wasm::Expression*, 10>; typename std::enable_if<(std::_PCC<true, _T1, _T2>::_ConstructiblePair<_U1, _U2>() && std::_PCC<true, _T1, _T2>::_ImplicitlyConvertiblePair<_U1, _U2>()), bool>::type <anonymous> = true; _T1 = wasm::Select* const; _T2 = wasm::SmallVector<wasm::Expression*, 10>]' at /usr/include/c++/13.2.1/bits/stl_pair.h:559:21,
    inlined from 'T& wasm::InsertOrderedMap<Key, T>::operator[](const Key&) [with Key = wasm::Select*; T = wasm::SmallVector<wasm::Expression*, 10>]' at /root/binaryen/src/support/insert_ordered.h:112:29,
    inlined from 'void wasm::Precompute::partiallyPrecompute(wasm::Function*)::StackFinder::visitSelect(wasm::Select*)' at /root/binaryen/src/passes/Precompute.cpp:472:24,
    inlined from 'static void wasm::Walker<SubType, VisitorType>::doVisitSelect(SubType*, wasm::Expression**) [with SubType = wasm::Precompute::partiallyPrecompute(wasm::Function*)::StackFinder; VisitorType = wasm::Visitor<wasm::Precompute::partiallyPrecompute(wasm::Function*)::StackFinder, void>]' at /root/binaryen/src/wasm-delegations.def:50:1:
/root/binaryen/src/support/small_vector.h:32:38: error: '<unnamed>.wasm::SmallVector<wasm::Expression*, 10>::fixed' may be used uninitialized [-Werror=maybe-uninitialized]
   32 | template<typename T, size_t N> class SmallVector {
      |                                      ^~~~~~~~~~~
In file included from /root/binaryen/src/passes/Precompute.cpp:38:
/root/binaryen/src/support/insert_ordered.h: In static member function 'static void wasm::Walker<SubType, VisitorType>::doVisitSelect(SubType*, wasm::Expression**) [with SubType = wasm::Precompute::partiallyPrecompute(wasm::Function*)::StackFinder; VisitorType = wasm::Visitor<wasm::Precompute::partiallyPrecompute(wasm::Function*)::StackFinder, void>]':
/root/binaryen/src/support/insert_ordered.h:112:29: note: '<anonymous>' declared here
  112 |     std::pair<const Key, T> kv = {k, {}};
      |                             ^~
At global scope:
cc1plus: note: unrecognized command-line option '-Wno-unknown-warning-option' may have been intended to silence earlier diagnostics
cc1plus: note: unrecognized command-line option '-Wno-implicit-int-float-conversion' may have been intended to silence earlier diagnostics
cc1plus: all warnings being treated as errors

This only happens on aarch64, we don't understand why and believe it's spurious. This PR intends to disable the specific warning only on aarch64. It fixes the compilations issue I could previously reproduce in the container used in CI, and a clean debian bullseye environment.