round: Abolished the CPU rounding mode change and modified it to a different logic.

[SakiTakamachi]

This doesn't happen with llvm 15.0.7, but I expect there are many users using 15.0.0, so I fixed it.
The llvm that comes with MacOS from the beginning may be 15.0.0, and to use 15.0.7 we need to install it from homebrew.

When I compared the assembly code, the one using llvm 15.0.0 was calculating places > 0 ? value * exponent : value / exponent before the CPU round mode change, so I made fix like this.

failed test:
https://github.com/php/php-src/actions/runs/7945635002/job/21692533874

testing in my repo:
https://github.com/SakiTakamachi/php-src/actions/runs/7968042924/job/21751687740

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#pragma STDC FENV_ACCESS ON is included in C99, but many compilers do not support it.
Based on that, I decided that the CPU rounding mode change feature should not be used even if it existed, and I modified it to a different logic that does not use it.

[nikic]

This code looks very broken. If you want to change the default floating-point environment in C, you must use #pragma STDC FENV_ACCESS ON, otherwise the compiler can and will assume the default environment.

[SakiTakamachi]

@nikic
Thank you! I didn't know this. My code looks like UB.

I'll fix it.

[SakiTakamachi]

@nikic
Many compilers do not support #pragma STDC FENV_ACCESS ON, so I modified the logic to not change the CPU rounding mode.

[SakiTakamachi]

@bukka

Could you please take a look at this?

[bukka]

I think it makes sense but didn't spend that much time to think about it. It's for master so should be fine.