druntime: redirect dual-ABI functions on glibc to IEEE128 version

[liushuyu]

This pull request redirects powerpc dual-ABI functions on glibc to the IEEE128 version if IEEE long double ABI is selected.

A new (reserved) version identifier, D_PPCUseIEEE128, is introduced to control the redirection behaviour. Compilers are expected to set this version identifier when the user selects IEEE 128 ABI for their project.

Required compiler changes (setting the D_PPCUseIEEE128 identifier) are also proposed in the following:

• LDC: ppc64(le): Add an option to use IEEE long double ABI on Linux ldc-developers/ldc#4833
• GDC: https://gcc.gnu.org/pipermail/gcc-patches/2025-February/675184.html

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[thewilsonator]

please use aliases instead of pragma(mangle, ...), like:

pragma(scanf)
     int __isoc99_scanf(scope const char* format, scope ...);
     ///
     alias scanf = __isoc99_scanf;
``

[liushuyu]

please use aliases instead of pragma(mangle, ...), like:

pragma(scanf)
     int __isoc99_scanf(scope const char* format, scope ...);
     ///
     alias scanf = __isoc99_scanf;
``

I don't know if I understand you correctly, but

extern (C) void a();

version (PPC64) {
    static if (real.mant_dig == 113) {
        extern (C) void a__ieee128();
        alias a = a__ieee128;
    }
}

void b() {
    a();
}

This does not work. b will still call a instead of a__ieee128 even if the conditions are met.
LDC and GDC also do not issue any warnings or errors in this case.

[liushuyu]

Assuming we can't do anything about the alias problem in the compiler, an alternative may be doing this (also taking the suggestions from the LDC pull request):

version (PPC64)
    enum PPCUseIEEE128 = real.mant_dig == 113;
else
    enum PPCUseIEEE128 = false;

static if (PPCUseIEEE128)
{
    real __strtoieee128(scope inout(char)* nptr, scope inout(char)** endptr);
    alias strtold = __strtoieee128;
}
else
{
    real strtold(scope inout(char)* nptr, scope inout(char)** endptr);
}

... which is a bit verbose for math.d as there are a lot of functions to be aliased.

[thewilsonator]

You only need to have one static if and one else branch, you can put multiple declarations for the different function is there.

[liushuyu]

You only need to have one static if and one else branch, you can put multiple declarations for the different function is there.

I know, but my concern is that declaring each function twice may increase maintenance burden (both versions have the same D function signature), as opposed to using mangle only needs one function definition