Support x64-only symbols on ARM64EC

[CaseyCarter]

• Adds support for building the STL for the ARM64EC platform. (No testing yet, just build.)
• Enable the __std_init_once_meow ALTERNATENAMEs in src/xonce2.cpp on ARM64EC. Note that we continue not to use the <mutex> ALTERNATENAMEs when compiling for ARM64EC - the linkage model is complicated - we only include the ALTERNATENAMEs to support linking with object files built targeting x64. For example, linking x64 static libraries into an ARM64EC binary needs to work.
• Isolate x86/x64-specific code in src/vector_algorithms.cpp so we can enable the non-vectorized fallback implementations for ARM64EC. Again, these aren't used when compiling for ARM64EC, but we need the symbols in the import lib to support linking with x64 object files.

Fixes #3382

Note that there's no test coverage here. We're close-but-not-ready to support testing of the STL on ARM64, but a ways away from being able to compile an object file for one platform and link it into a binary built for another. I did validate the fix on an ARM64 VM using files test.cpp:

#include <cassert>
#include <cstddef>

extern int f(const char* ptr, std::size_t length);
extern int g();

int main() {
    static const char data[] = "this is a test, this is only a test";
    assert(f(data, sizeof(data)) == 14);
    assert(g() == 42);
}

and repro.cpp:

#include <algorithm>
#include <cstddef>
#include <mutex>

int f(const char* ptr, std::size_t length) {
    return std::find(ptr, ptr + length, ',') - ptr;
}

int g() {
    static std::once_flag nonce;
    static int x = 0;
    std::call_once(nonce, [] { x = 42; });
    return x;
}

compiling repro.cpp for x64, and linking the resulting object with test.cpp (EDIT: built with /arm64EC, of course). I performed this process once for each of /MT, /MTd, /MD, and /MDd and verified the resulting program linked and ran successfully. (Debug should make no difference, but paranoia never hurts.)

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for figuring out how to fix this major bug that was affecting several customers! 🛠️ 💡 😻

[kobykahane]

Will this be backported to 17.6/17.7?

[StephanTLavavej]

At this time, we have no plans to backport the fix. There's an escape hatch of disabling the vectorized algorithms when compiling the x64 object files.

[kobykahane]

At this time, we have no plans to backport the fix. There's an escape hatch of disabling the vectorized algorithms when compiling the x64 object files.

Thanks @StephanTLavavej. Unfortunately, in our case, we're not in a position to recompile the x64 static libraries we're attempting to consume in our ARM64EC build. If we were, we'd probably just rebuild them as ARM64 and wouldn't have opted for EC. Additionally, we're hitting the __std_init_once_* cases, to which disabling the vectorized algorithms doesn't seem to apply (although it seems that manually defining the alternate names could take care of that one).