Optimize find for ARM64EC

[AlexGuteniev]

I'm not completely sure what exactly ARM64EC is (#2740).
But the upstream memchr / wmemchr are apparently properly optimized for it.

I didn't benchmark or test this change. It seems to compile though.

[YexuanXiao]

Here is a technical note written in 2024 about ARM64EC and x64 emulation. For writing C++ code, the key points are: ARM64EC is aarch64 code. However, during preprocessing, it masquerades as x64 through macros, not ARM64. Therefore, if anyone wants to optimize ARM64EC performance, they only need to be careful not to use AMD64-specific vector intrinsics, as these are provided by softintrin.lib.

[AlexGuteniev]

Here is a technical note written in 2024 about ARM64EC and x64 emulation

Thanks, I now see that ARM64 intrinsics are the way to go, also no AVX+ intrinsics are expected.

Still, I'm not sure how bad the emulation is, so not sure if emulated SSE4.2 is neccessarily worse than native scalar.

[Copilot]

Pull Request Overview

This PR aims to optimize the find algorithms for the ARM64EC platform by using the optimized C-library functions memchr and wmemchr.

• Introduces an ARM64EC-specific branch in __std_find_trivial_1 using memchr.
• Introduces an ARM64EC-specific branch in __std_find_trivial_2 using wmemchr with type casts and a modified element count.